Fix SubDomainSet bug

[rhodrin]

Fixes a bug that meant that equations assocciated with a SubDomainSet had to be ordered in 'groups'. Fixes issue #1453. Thanks to @mloubout.

[codecov]

Codecov Report

Merging #1457 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1457   +/-   ##
=======================================
  Coverage   86.30%   86.31%           
=======================================
  Files         193      193           
  Lines       27865    27897   +32     
  Branches     3787     3786    -1     
=======================================
+ Hits        24049    24078   +29     
- Misses       3388     3390    +2     
- Partials      428      429    +1     

Impacted Files	Coverage Δ
tests/test_data.py	97.61% <ø> (ø)
devito/data/data.py	94.44% <100.00%> (ø)
devito/operator/operator.py	87.83% <100.00%> (-0.09%)	⬇️
tests/test_subdomains.py	100.00% <100.00%> (ø)
devito/compiler.py	54.36% <0.00%> (-0.68%)	⬇️
devito/types/grid.py	93.11% <0.00%> (-0.37%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 994aff7...af326c3. Read the comment docs.

[FabioLuporini]

I love it when we fix bugs by actually dropping code :)

[FabioLuporini]

merged, thanks

[mloubout]

As an aside, some "cleanup" may be done at some point in the generated C code that now generates the implicit expressions for each equation and therefore split in two loops if there is two equations.