Action of MatrixGroup on a MPolynomialRing (first attempt, changes likely necessary!)

[Bruno-TT]

Attempting to revive dead ticket #4513

Not too familiar with the surrounding code so this is probably very imperfect - please let me know what to change!

[github-actions]

Documentation preview for this PR is ready! 🎉
Built with commit: d5fe6fd

[tscrim]

It would be good to have this, but I have a number of issues with the current proposal:

• I don't think __call__ of a group element of a matrix is the right place to implement this. I think we should use some other symbol for this different (but natural) action. For example, @ (matmul) or ^ (either xor or pow). This would be more natural IMO. It would also allow us to fit it into the coercion framework.
• Does this action extend to all matrices? I thought it did.
• I don't think mRingPoly.variables() is returning what you expect (all variables of its parent):

sage: R.<x,y,z> = QQ[]
sage: x + y
x + y
sage: _.variables()
(x, y)

If you do want to use the polynomial rather than the parent, then you aren't really implementing an action.

• PEP8: in particular, Python uses lowercase_underscore variables.
• Rather than using the variables as a matrix, you should use them as a vector.
• You are using both other and mRingPoly`. I see no reason to assign another variable.
• Use isinstance(x, Foo) instead of type(x) == Foo to allow for subclasses.