Implement __getitem__ and alias weight methods for quasimodular forms

[DavidAyotte]

📚 Description

This PR implements a __getitem__ method for quasimodular forms ring elements to access their components of a fixed degree. Moreover, the PR defines the alias degree = weight (which is the right nomenclature for elements of a graded algebra, as discussed with @fchapoton).

@videlec

📝 Checklist

• I have made sure that the title is self-explanatory and the description concisely explains the PR.
• I have linked an issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation accordingly.

⌛ Dependencies

[videlec]

Where the convention about homogeneous_component is actually taken from?

$ git grep -l homogeneous_component categories/*py
categories/filtered_modules_with_basis.py
categories/finite_dimensional_graded_lie_algebras_with_basis.py
categories/finite_dimensional_lie_algebras_with_basis.py

None of these categories match the quasi-modular forms.

[videlec]

Where the convention about homogeneous_component is actually taken from?

$ git grep -l homogeneous_component categories/*py
categories/filtered_modules_with_basis.py
categories/finite_dimensional_graded_lie_algebras_with_basis.py
categories/finite_dimensional_lie_algebras_with_basis.py

None of these categories match the quasi-modular forms.

All right. As discussed with David, the homogeneous_component that appears in the patch comes from the quasi-modular forms and is not introduce here.

[DavidAyotte]

Thank you!