added __version__ attribute

[RemDelaporteMathurin]

Proposed changes

This PR fixes #489 by adding a version attribute to FESTIM

Types of changes

What types of changes does your code introduce to FESTIM?

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Code refactoring
• Documentation Update (if none of the other choices apply)
• New tests

Checklist

• Black formatted
• Unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...

[codecov]

Codecov Report

Merging #491 (8adaf12) into dev (67b34e0) will decrease coverage by 0.09%.
The diff coverage is 81.81%.

@@            Coverage Diff             @@
##              dev     #491      +/-   ##
==========================================
- Coverage   98.89%   98.80%   -0.10%     
==========================================
  Files          54       54              
  Lines        1994     2005      +11     
==========================================
+ Hits         1972     1981       +9     
- Misses         22       24       +2     

Impacted Files	Coverage Δ
FESTIM/__init__.py	97.10% <81.81%> (-2.90%)	⬇️

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[RemDelaporteMathurin]

can't do much about the coverage I'm afraid