dsl: Make CondEq and CondNe easier to import

[EdCaunt]

Allow import of CondEq and CondNe from devito rather than requiring devito.symbolics. In due course, these should probably move to devito.types.relational and use the AbstractRel mixin.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.69%. Comparing base (0597d00) to head (acfe740).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2425   +/-   ##
=======================================
  Coverage   86.69%   86.69%           
=======================================
  Files         235      235           
  Lines       44750    44764   +14     
  Branches     8296     8298    +2     
=======================================
+ Hits        38796    38810   +14     
  Misses       5226     5226           
  Partials      728      728           

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[mloubout]

need rebase