Check that conditions and numerical operators aren't mixed

[MichaelClerx]

• Remove obsolete exception import check. Closes Move error import check to unit test #1064.
• Check types in constructors of all expressions. Closes Expression validation: Check for appropriate Condition/not-a-Condition types? #941.
• Update expression tests
• Update parsing tests: exception should propagate with nice error message
• Check type in Variable.set_rhs No. do in validate like others
• Check type in Variable._set_initial_value No. do in validate like rest

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (e0b98fd) to head (36f096f).

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1075   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          100       100           
  Lines        20839     20844    +5     
=========================================
+ Hits         20839     20844    +5     

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