dsl: prevent aggregation for symbolic coefficients

[mloubout]

Fixes #2181

[codecov]

Codecov Report

Merging #2182 (3b9091a) into master (bc4d02d) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2182      +/-   ##
==========================================
- Coverage   87.09%   87.09%   -0.01%     
==========================================
  Files         226      226              
  Lines       40160    40172      +12     
  Branches     7329     7330       +1     
==========================================
+ Hits        34979    34987       +8     
- Misses       4602     4605       +3     
- Partials      579      580       +1     

Files Changed	Coverage Δ
devito/parameters.py	85.10% <100.00%> (ø)
devito/passes/equations/linearity.py	100.00% <100.00%> (ø)
tests/test_symbolic_coefficients.py	99.10% <100.00%> (+0.04%)	⬆️

... and 2 files with indirect coverage changes