dsl: Introduce ability to define Functions on Subdomains

[EdCaunt]

Add Functions on SubDomains functionality.

Remaining todo (for future PRs I think):

• MPI in full mode + functions on subdomains leads to an error which I haven't had time to debug

[codecov]

Codecov Report

Attention: Patch coverage is 97.47145% with 31 lines in your changes missing coverage. Please review.

Project coverage is 87.52%. Comparing base (34dba05) to head (3b8ec13).
Report is 35 commits behind head on master.

Files with missing lines	Patch %	Lines
devito/types/grid.py	84.15%	14 Missing and 2 partials ⚠️
devito/mpi/distributed.py	95.12%	4 Missing and 2 partials ⚠️
devito/types/sparse.py	20.00%	4 Missing ⚠️
devito/data/decomposition.py	97.29%	0 Missing and 1 partial ⚠️
devito/deprecations.py	75.00%	1 Missing ⚠️
tests/test_interpolation.py	99.55%	0 Missing and 1 partial ⚠️
tests/test_mpi.py	98.82%	0 Missing and 1 partial ⚠️
tests/test_subdomains.py	99.79%	1 Missing ⚠️

Additional details and impacted files

@@ Coverage Diff @@ ## master #2245 +/- ## ========================================== + Coverage 87.29% 87.52% +0.22%  ========================================== Files 238 238 Lines 46063 47034 +971 Branches 4080 4144 +64 ========================================== + Hits 40211 41165 +954  - Misses 5162 5176 +14  - Partials 690 693 +3 

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[FabioLuporini]

For the amount of changes introduced, I'd prefer to see a lot more new tests.

Shouldn't all (most) of the old tests that @rhodrin wrote be here as well?

[mloubout]

quick pass