Implement band math endpoint

[dionhaefner]

No description provided.

[codecov]

Codecov Report

Merging #128 into master will increase coverage by 0.11%.
The diff coverage is 98.42%.

@@            Coverage Diff             @@
##           master     #128      +/-   ##
==========================================
+ Coverage   97.35%   97.47%   +0.11%     
==========================================
  Files          39       42       +3     
  Lines        1931     2095     +164     
  Branches      255      275      +20     
==========================================
+ Hits         1880     2042     +162     
- Misses         27       28       +1     
- Partials       24       25       +1

Impacted Files	Coverage Δ
terracotta/server/keys.py	100% <100%> (ø)	⬆️
terracotta/server/rgb.py	96.22% <100%> (-0.07%)	⬇️
terracotta/server/datasets.py	100% <100%> (ø)	⬆️
terracotta/handlers/compute.py	100% <100%> (ø)
terracotta/server/compute.py	100% <100%> (ø)
terracotta/server/singleband.py	100% <100%> (ø)	⬆️
terracotta/scripts/connect.py	93.33% <100%> (ø)	⬆️
terracotta/server/colormap.py	97.29% <100%> (-0.14%)	⬇️
terracotta/server/metadata.py	100% <100%> (ø)	⬆️
terracotta/server/flask_api.py	89.39% <92.85%> (+0.5%)	⬆️
... and 4 more

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[dionhaefner]

@mrpgraae @j08lue @nmheim Could I ask one of you to take over from here? The feature is done from my perspective, but there are still some architectural decisions to be made (#20), the handler and flask route need test coverage, and the endpoint has to be documented.

[dionhaefner]

Done from my side. Needs more documentation, but so do all endpoints.