compiler: Patch symbolic coefficients over cross derivatives

[codecov[bot]]

Codecov Report

Merging #2248 (a30b392) into master (4b0e39a) will increase coverage by 0.02%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2248      +/-   ##
==========================================
+ Coverage   86.88%   86.90%   +0.02%     
==========================================
  Files         229      229              
  Lines       41691    41759      +68     
  Branches     7696     7703       +7     
==========================================
+ Hits        36223    36292      +69     
- Misses       4839     4840       +1     
+ Partials      629      627       -2     

Files	Coverage Δ
devito/finite_differences/coefficients.py	93.83% <100.00%> (+0.70%)	:arrow_up:
devito/finite_differences/differentiable.py	94.12% <100.00%> (+0.21%)	:arrow_up:
devito/types/dense.py	93.88% <ø> (-0.04%)	:arrow_down:
tests/test_symbolic_coefficients.py	99.28% <100.00%> (+0.15%)	:arrow_up:
tests/test_unexpansion.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes