Group symbols & labels so they can be easily styled

[hyanwong]

This allows us to do e.g. in a jupyter notebook:

display(HTML('<style>svg #labels .mutations {display: none}</style>' + ts.draw_svg()))

to turn the mutation labels off. Similarly for symbols, etc etc.

[codecov]

Codecov Report

Merging #241 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #241      +/-   ##
==========================================
+ Coverage   86.58%   86.58%   +<.01%     
==========================================
  Files          16       16              
  Lines        9389     9393       +4     
  Branches     1704     1704              
==========================================
+ Hits         8129     8133       +4     
  Misses        750      750              
  Partials      510      510

Flag	Coverage Δ
#c_tests	86.58% <100%> (ø)	⬆️
#python_tests	98.21% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
python/tskit/drawing.py	94.9% <100%> (+0.03%)	⬆️

---

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[codecov]

Codecov Report

Merging #241 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #241      +/-   ##
==========================================
+ Coverage   86.58%   86.58%   +<.01%     
==========================================
  Files          16       16              
  Lines        9389     9393       +4     
  Branches     1704     1704              
==========================================
+ Hits         8129     8133       +4     
  Misses        750      750              
  Partials      510      510

Flag	Coverage Δ
#c_tests	86.58% <100%> (ø)	⬆️
#python_tests	98.21% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
python/tskit/drawing.py	94.9% <100%> (+0.03%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7668a68...73b2347. Read the comment docs.

[codecov]

Codecov Report

Merging #241 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #241      +/-   ##
==========================================
+ Coverage   86.58%   86.58%   +<.01%     
==========================================
  Files          16       16              
  Lines        9389     9393       +4     
  Branches     1704     1704              
==========================================
+ Hits         8129     8133       +4     
  Misses        750      750              
  Partials      510      510

Flag	Coverage Δ
#c_tests	86.58% <100%> (ø)	⬆️
#python_tests	98.21% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
python/tskit/drawing.py	94.9% <100%> (+0.03%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7668a68...73b2347. Read the comment docs.

[codecov]

Codecov Report

Merging #241 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #241      +/-   ##
==========================================
+ Coverage   86.58%   86.58%   +<.01%     
==========================================
  Files          16       16              
  Lines        9389     9393       +4     
  Branches     1704     1704              
==========================================
+ Hits         8129     8133       +4     
  Misses        750      750              
  Partials      510      510

Flag	Coverage Δ
#c_tests	86.58% <100%> (ø)	⬆️
#python_tests	98.21% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
python/tskit/drawing.py	94.9% <100%> (+0.03%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7668a68...73b2347. Read the comment docs.

[codecov]

Codecov Report

Merging #241 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #241      +/-   ##
==========================================
+ Coverage   86.58%   86.58%   +<.01%     
==========================================
  Files          16       16              
  Lines        9389     9393       +4     
  Branches     1704     1704              
==========================================
+ Hits         8129     8133       +4     
  Misses        750      750              
  Partials      510      510

Flag	Coverage Δ
#c_tests	86.58% <100%> (ø)	⬆️
#python_tests	98.21% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
python/tskit/drawing.py	94.9% <100%> (+0.03%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7668a68...73b2347. Read the comment docs.

[jeromekelleher]

LGTM, thanks!