implement the depth of a quasimodular form

[DavidAyotte]

📚 Description

This PR implements the depth of a quasimodular form. Recall that the depth of a quasimodular form $f$ of weight $k$ is the integer $p$ such that:

$$ f = f_0 + f_1 E_2 + \cdots + f_p E_2^p $$

where $f_i$ is of weight $k - 2*i$ and $f_p\neq 0$.

CC: @videlec

📝 Checklist

• [ x] The title is concise, informative, and self-explanatory.
• [x ] The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion. (not applicable)
• [x ] I have created tests covering the changes.
• [x ] I have updated the documentation accordingly.

⌛ Dependencies

[grhkm21]

Small changes (just "homogeneous" is correct, unless what you wrote has a special meaning, but I don't think so?). Feel free to reject the suggestions

Otherwise looks good to me!

[grhkm21]

This should be the final change.

[grhkm21]

Looks good! Thank you :)

[github-actions]

Documentation preview for this PR (built with commit 97286da; changes) is ready! 🎉

[DavidAyotte]

Looks good! Thank you :)

Thank you for reviewing my PRs!