Add MacOS+Windows tests; clean up .gitignore

[yannikschaelte]

• Execute CI unit tests on macos and windows in addition to ubuntu
• Add all outputs to .gitignore

Fixes #41, #89

[codecov-io]

Codecov Report

Merging #115 (4d4249c) into develop (3c45897) will increase coverage by 10.86%.
The diff coverage is 90.78%.

@@             Coverage Diff              @@
##           develop     #115       +/-   ##
============================================
+ Coverage    77.85%   88.72%   +10.86%     
============================================
  Files            5        6        +1     
  Lines          429      461       +32     
============================================
+ Hits           334      409       +75     
+ Misses          95       52       -43     

Impacted Files	Coverage Δ
yaml2sbml/YamlModel.py	84.00% <77.77%> (-0.49%)	⬇️
yaml2sbml/yaml_validation.py	89.65% <77.77%> (+17.65%)	⬆️
yaml2sbml/yaml2sbml.py	89.86% <92.10%> (+14.48%)	⬆️
yaml2sbml/__init__.py	100.00% <100.00%> (ø)
yaml2sbml/version.py	100.00% <100.00%> (ø)
yaml2sbml/yaml2PEtab.py	94.17% <100.00%> (+23.46%)	⬆️

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