feat: add resultant for list of multivariate polynomials

[thofma]

I need it for an application and I think it would be nice to have. There are more algorithms that one could implement, but this one works fine for my application.

@wdecker maybe you could have a look?

[codecov]

Codecov Report

Merging #3546 (9d71e8c) into master (d367722) will increase coverage by 0.03%.
Report is 25 commits behind head on master.
The diff coverage is 97.56%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3546      +/-   ##
==========================================
+ Coverage   82.16%   82.20%   +0.03%     
==========================================
  Files         568      569       +1     
  Lines       77055    77291     +236     
==========================================
+ Hits        63312    63534     +222     
- Misses      13743    13757      +14     

Files	Coverage Δ
src/Rings/resultant.jl	97.56% <97.56%> (ø)

... and 41 files with indirect coverage changes

[fieker]

Can someone checke the math? The wiki page wants homogenous polynomials, but this is not checked (by the looks: deliberatey not)
Should this apply to graded polynomials (only)? Standart grading? Any grading?

[thofma]

It must be homogeneous with respect to the standard grading. It is not checked, because it cannot be checked with the current state of OSCAR:

julia> K = algebraic_closure(QQ); Kxy, (x, y) = K["x", "y"]; is_homogeneous(x)
true

julia> K = QQ; Kxy, (x, y) = K["x", "y"]; is_homogeneous(x)
ERROR: MethodError: no method matching is_homogeneous(::QQMPolyRingElem)

[fingolfin]

Nemocas/AbstractAlgebra.jl#1649 adds is_homogeneous for all MPolyRingElem

[thofma]

I remembered that we have an _is_homogeneous for this purpose. I am using this and added the test. This should be good to go now.