avoid sympy bug in ast_parser

[arabidopsis]

Currently sympy has a bug that means that its ast_parser won't work on
pythons >= 3.5.

Also the parser will not work with gene IDs that have splice variants (i.e. they have a '.').

Solution: use cobra's own parse_gpr and then transform to a sympy expression using a NodeTransformer.

[Midnighter]

Hi, thank you for answering on #19 and making this PR. The code that you're introducing looks solid to me. It could do with a docstring on the Transform class which just summarizes the intention in one line.

I'm happy to merge this now but please be aware that we haven't actively developed the driven code in a while. We will do so, however, until the middle of the year and also release it properly on PyPI.

[codecov-io]

Codecov Report

Merging #20 into devel will decrease coverage by 8.78%.
The diff coverage is 27.27%.

@@            Coverage Diff            @@
##           devel      #20      +/-   ##
=========================================
- Coverage     35%   26.22%   -8.79%     
=========================================
  Files         23       23              
  Lines        677      694      +17     
=========================================
- Hits         237      182      -55     
- Misses       440      512      +72

Impacted Files	Coverage Δ
src/driven/data_sets/expression_profile.py	46.06% <27.27%> (-11.7%)	⬇️
src/driven/omics/imat.py	17.64% <0%> (-79.42%)	⬇️
src/driven/omics/gimme.py	25% <0%> (-70.84%)	⬇️

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