switch to Entity.__dict__ instead of __context__

[gilesknap]

Tidy up the approach of passing Entity attributes to Jinja context. Including how 'values' fields are added.

All values for the jinja context are now just ordinary attributes on Entity and area passed using dict.

[gilesknap]

@coretl I think I'm happy with this now. Take a look if you wish.

[codecov-commenter]

Codecov Report

Merging #69 (5880bfe) into main (6751512) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##             main      #69      +/-   ##
==========================================
- Coverage   97.40%   97.38%   -0.03%     
==========================================
  Files           9        9              
  Lines         386      382       -4     
==========================================
- Hits          376      372       -4     
  Misses         10       10              

Impacted Files	Coverage Δ
src/ibek/ioc.py	97.75% <100.00%> (-0.08%)	⬇️
src/ibek/render.py	96.47% <100.00%> (-0.09%)	⬇️
src/ibek/utils.py	90.90% <100.00%> (+0.28%)	⬆️

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