Allow adding and removing coefficients from Polynomial

[henrikjacobsenfys]

Allow adding and removing coefficients from Polynomial

Closes #63

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.99%. Comparing base (45e4757) to head (5c1b03b).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #212   +/-   ##
========================================
  Coverage    98.98%   98.99%           
========================================
  Files           53       53           
  Lines         4037     4060   +23     
  Branches       697      706    +9     
========================================
+ Hits          3996     4019   +23     
  Misses          19       19           
  Partials        22       22           

Flag	Coverage Δ
unittests	98.99% <100.00%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
...easydynamics/sample_model/components/polynomial.py	100.00% <100.00%> (ø)

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[henrikjacobsenfys]

Rebased onto the new develop and updated to the new unit API (following the Unit overhaul 1-6 merge):

• \unit=\ -> \x_unit=/\y_unit=\ throughout; sparse-dict init now preserves both.
• Kept develop's existing \Parameter\ support in _init_\ and the \coefficients\ setter. The earlier revision of this branch dropped it, which would have been an unrelated regression.
• \�dd_coefficient\ /
  emove_coefficient\ and sparse-dict construction carried over unchanged, plus tests for interaction with \convert_x_unit\ and \get_all_variables.
• Dict keys now reject \�ool\ explicitly (\�ool\ is a subclass of \int).
• Removed the {'coefficients': {}}\ case from \ est_input_type_validation_raises: an empty dict is now a valid-but-empty dict raising \ValueError, covered by \ est_sparse_dict_empty_raises.

The previous CI failure was \ est_repr\ expecting the
ame = X\ repr format, which the branch was too old to have. That is resolved by the rebase.