feat: biorbd parsing pi expressions

[Ipuch]

i couldn't parse my model because of pi/2 no detected as float

[codecov-commenter]

Codecov Report

❌ Patch coverage is 48.97959% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.90%. Comparing base (c25892f) to head (8b9c19e).
⚠️ Report is 148 commits behind head on main.

Files with missing lines	Patch %	Lines
biobuddy/model_parser/biorbd/utils.py	29.16%	17 Missing ⚠️
...iobuddy/model_parser/biorbd/biomod_model_parser.py	68.00%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #113      +/-   ##
==========================================
- Coverage   91.12%   90.90%   -0.22%     
==========================================
  Files          98      102       +4     
  Lines        8234     9255    +1021     
==========================================
+ Hits         7503     8413     +910     
- Misses        731      842     +111     

Flag	Coverage Δ
unittests	90.90% <48.97%> (-0.22%)	⬇️

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