Add support for pseudomorphisms

[xcaruso]

This PR implements pseudomorphisms.

Let $M, M'$ be modules over a ring $R$, $\theta: R \to R$ be a ring homomorphism, and $\delta: R \to R$ be a $\theta$-derivation, which is a map such that $\delta(xy) = \theta(x)\delta(y) + \delta(x)y$.
A pseudomorphism $f : M \to M$ is an additive map such that $f(\lambda x) = \theta(\lambda)f(x) + \delta(\lambda) x$ for all $\lambda$ and $x$.

This PR is based on a former PR by @ymusleh (that I could not find, I don't know why).

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.

⌛ Dependencies

[github-actions]

Documentation preview for this PR (built with commit 1db04c2; changes) is ready! 🎉
This preview will update shortly after each push to this PR.

[xcaruso]

There seems to be a genuine doctest failure in CI.

Yes. I committed some changes in the file matrix_cdv.pyx by mistake (it was a work in progress for another PR). Sorry for this.

[xcaruso]

I tried to fix the test suite. Doing this, I noticed some problems with categories; they are now fixed.

[xcaruso]

@kryzar Please tell me if you have some comment on this PR.

[kryzar]

Here are some suggestions to enhance the doc: