Utilise PetscSection to constrain essential boundary conditions

[ZoeLeibowitz]

Not ready for review

[codecov]

Codecov Report

❌ Patch coverage is 29.42207% with 403 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.56%. Comparing base (a91bdaf) to head (cef2e7f).

Files with missing lines	Patch %	Lines
tests/test_petsc.py	16.52%	101 Missing ⚠️
devito/petsc/iet/callbacks.py	13.04%	80 Missing ⚠️
devito/petsc/types/dimension.py	35.59%	38 Missing ⚠️
devito/petsc/equations.py	24.48%	36 Missing and 1 partial ⚠️
devito/petsc/iet/passes.py	24.39%	30 Missing and 1 partial ⚠️
devito/petsc/iet/builder.py	26.47%	25 Missing ⚠️
devito/petsc/iet/type_builder.py	13.63%	19 Missing ⚠️
devito/petsc/types/metadata.py	44.11%	19 Missing ⚠️
devito/petsc/types/object.py	68.96%	18 Missing ⚠️
devito/data/decomposition.py	5.88%	16 Missing ⚠️
... and 6 more

Additional details and impacted files

@@            Coverage Diff             @@
##            petsc    #2844      +/-   ##
==========================================
- Coverage   76.00%   75.56%   -0.44%     
==========================================
  Files         273      275       +2     
  Lines       54428    54875     +447     
  Branches     4603     4633      +30     
==========================================
+ Hits        41367    41467     +100     
- Misses      12225    12572     +347     
  Partials      836      836              

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