Add evaluate: bool argument to math parser

[dweindl]

So far, when math expressions were parsed, they were evaluated as far as possible. This was not always desirable.

Now, this optional as far as conveniently possible.

Closes #363.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 94.44444% with 1 line in your changes missing coverage. Please review.

Project coverage is 74.57%. Comparing base (101c79c) to head (27a2b28).
Report is 20 commits behind head on develop.

Files with missing lines	Patch %	Lines
petab/v1/math/SympyVisitor.py	93.75%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #365      +/-   ##
===========================================
+ Coverage    74.55%   74.57%   +0.02%     
===========================================
  Files           57       57              
  Lines         6075     6080       +5     
  Branches      1057     1058       +1     
===========================================
+ Hits          4529     4534       +5     
  Misses        1141     1141              
  Partials       405      405              

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[dilpath]

Looks good 👍