gh-36128: Compute the characteristic varieties of a finitely presented group
    
<!-- Describe your changes here in detail -->
Let $G$ be a finitely presented group. For a field $\mathbb{k}$, let
$\Lambda:=\mathbb{k}[G]$ be its group algebra and let $\mathbb{T}$ be
its maximal spec, which is a torus. An element $\xi\in\mathbb{T}$
determines a local system of coefficients and the characteristic
varieties of $G$ are defined as
$$V_k(G):=\\{\xi\in\mathbb{T}\mid \dim H^1(G;\xi)\geq k\\}.$$
Except for the neutral element these are the reduced varieties of the
Fitting ideals of the Alexander matrix of the presentation. The function
`char_var` gives systems of generators of these ideals. A `groebner`
option is given to give Groebner basis; it is set as an option since the
computations may be quite long.

### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->
<!-- If your change requires a documentation PR, please link it
appropriately -->
<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
<!-- Feel free to remove irrelevant items. -->

- [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.
- [x] I have created tests covering the changes.
- [x] I have updated the documentation accordingly.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on
- #12345: short description why this is a dependency
- #34567: ...
-->

<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
    
URL: #36128
Reported by: Enrique Manuel Artal Bartolo
Reviewer(s): Enrique Manuel Artal Bartolo, miguelmarco, Travis Scrimshaw

Author: Release Manager

src/doc/en/reference/references/index.rst

@@ -1388,6 +1388,10 @@ REFERENCES:
              20(1.6):1-18, 2015. :doi:`10.1145/2780652` or
              [`<https://hal.inria.fr/hal-01182890>`_].
 
+.. [CS1999a] \D. Cohen and A. Suciu. *Characteristic varieties of arrangements*.
+             Math. Proc. Cambridge Philos. Soc.127 (1999), no.1, 33–-53.
+             :doi:`10.1017/S0305004199003576`.
+
 .. [CCLSV2005] \M. Chudnovsky, G. Cornuejols, X. Liu, P. Seymour, K. Vuskovic.
                *Recognizing berge graphs*.
                Combinatorica vol 25 (2005), n 2, pages 143--186.