Added enhancement for power series that allows access to the coefficients of specific terms

[Noel-Roemmele]

Fixes #39314. Adds an enhancement to power series that when given a tuple of values corresponding to the powers of the term gives you the coefficient for said term. Functionality is similar to how coefficients are retrieved for multivariable polynomials. Also added a test for the enhancement.

📝 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

[DaveWitteMorris]

Thanks for the enhancement. I will set to positive review on Wednesday if there are no other comments and make ptestlong (or CI testing) does not uncover any problems.

However, I think the initial docstring can be clarified. Perhaps something like:

Return the coefficient of the monomial ``x1^e1 * x2^e2 * ... * xk^ek`` 
if ``n = (e_1, e2, ..., ek)`` is a tuple whose length is the number of variables 
``x1,x2,...,xk`` in the power series ring. 

Return the sum of the monomials of degree ``n`` if ``n`` is an integer.

[github-actions]

Documentation preview for this PR (built with commit 97cb9fb; changes) is ready! 🎉
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[DaveWitteMorris]

Sorry, I thought I already told you this, but it's not in my comment above, so apparently not: you need to change :trac" to :issue: in line 698.

[mantepse]

Suggested change

	Ensure that the enhancment detailed in :issue:`39314` works as intended::
	Ensure that the enhancement detailed in :issue:`39314` works as intended::

[mantepse]

shouldn't that be indented further? please check!

[mantepse]

I'm a bit split on this solution, because it makes power series behave differently from everything else. Did you think about the possibility of a more generic solution?

It would be good to at least try to unify the interface for polynomials, power series and lazy power series.