Label get param

[BalzaniEdoardo]

This PR addresses the following

1. Add a __getitem__ to Basis. Labels can be used to get the desired basis in the tree.
2. Improved label logic:
   • User provided labels in composite basis must be unique.
   • Default labels in composite bases (class name) is disambiguated by appending _<number>.
   • Labeling a basis using another basis type class name results in an error.
3. get_params and set_params override default behavior: instead of reflecting the full attribute structure, basis1__basis2...__parameter_name, the parameter key is set to <label>_parameter_name.

For a concrete example, refer to the new how-to note: handling_composite_bases.md

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.39344% with 11 lines in your changes missing coverage. Please review.

Project coverage is 97.08%. Comparing base (07a9b22) to head (67f7548).
Report is 5 commits behind head on development.

Files with missing lines	Patch %	Lines
src/nemos/basis/_basis_mixin.py	96.62%	6 Missing ⚠️
src/nemos/basis/_basis.py	96.19%	4 Missing ⚠️
src/nemos/basis/_transformer_basis.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           development     #306      +/-   ##
===============================================
- Coverage        97.40%   97.08%   -0.33%     
===============================================
  Files               35       35              
  Lines             2854     3120     +266     
===============================================
+ Hits              2780     3029     +249     
- Misses              74       91      +17     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.