Functions on subdomains (WIP)

[rhodrin]

This PR adds the ability to define Function's on SubDomain's, e.g.

f = Function(name='f', grid=grid, subdomain=grid.subdomains['middle'])

The shape of the Data allocated will then be grid.subdomains['middle'].shape (or grid.subdomains['middle'].shape_local on each rank with MPI).

NOTE: Submitting for testing. Lots of changes and refactoring are still required.

[codecov[bot]]

Codecov Report

Merging #1380 into master will decrease coverage by 0.24%. The diff coverage is 95.14%.

@@            Coverage Diff             @@
##           master    #1380      +/-   ##
==========================================
- Coverage   87.00%   86.75%   -0.25%     
==========================================
  Files         188      188              
  Lines       27354    26257    -1097     
  Branches     3712     3740      +28     
==========================================
- Hits        23799    22779    -1020     
+ Misses       3126     3048      -78     
- Partials      429      430       +1     

Impacted Files	Coverage Δ
devito/types/grid.py	91.05% <85.50%> (-2.43%)	:arrow_down:
devito/ir/equations/algorithms.py	100.00% <100.00%> (ø)
devito/operations/interpolators.py	90.85% <100.00%> (+0.67%)	:arrow_up:
devito/types/dense.py	93.14% <100.00%> (-0.54%)	:arrow_down:
tests/test_caching.py	99.74% <100.00%> (-0.01%)	:arrow_down:
tests/test_subdomains.py	100.00% <100.00%> (ø)
devito/ir/support/stencil.py	51.56% <0.00%> (-3.51%)	:arrow_down:
tests/test_gpu_openacc.py	34.17% <0.00%> (-3.18%)	:arrow_down:
examples/seismic/utils.py	74.78% <0.00%> (-3.08%)	:arrow_down:
tests/test_tensors.py	84.26% <0.00%> (-2.14%)	:arrow_down:
... and 106 more

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