Generic exterior algebra and relative BGG

[HechtiDerLachs]

This is a rebase of #4097 .

[HechtiDerLachs]

I was looking into how to not widen the signatures of AdmissibleRingElems for the ModuleFPs to all of NCRingElem, but failed for the following reasons. So far, we declare the AdmissibleRingElems as const. That means we have to include all types we would like to put into this union up front. In the case of ExtAlgElem and its parent, this is bad, because they use FreeMods in their internals! So the cat bites its tail here and I haven't yet figured out a good way to resolve this.

Either way, we need to resolve this before merging!

[HechtiDerLachs]

I guess, I should still add some more comments and documentation. But I would like to keep this PR in the loop, as it is a take on one project which has explicitly been mentioned in the SFB application and I presume that there is some interest in taking things forward here.

[codecov]

Codecov Report

Attention: Patch coverage is 86.81447% with 113 lines in your changes missing coverage. Please review.

Project coverage is 84.38%. Comparing base (3af3c91) to head (9351670).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
...xperimental/ExteriorAlgebra/src/ExteriorAlgebra.jl	85.47%	87 Missing ⚠️
experimental/ExteriorAlgebra/src/BGG_complex.jl	88.23%	26 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4360      +/-   ##
==========================================
+ Coverage   84.30%   84.38%   +0.07%     
==========================================
  Files         651      653       +2     
  Lines       86483    87339     +856     
==========================================
+ Hits        72913    73701     +788     
- Misses      13570    13638      +68     

Files with missing lines	Coverage Δ
experimental/ExteriorAlgebra/src/Types.jl	100.00% <100.00%> (ø)
experimental/ExteriorAlgebra/test/runtests.jl	95.08% <100.00%> (+4.17%)	⬆️
src/Modules/ModuleTypes.jl	78.90% <ø> (ø)
src/Modules/ModulesGraded.jl	73.96% <ø> (+0.46%)	⬆️
src/Oscar.jl	69.51% <100.00%> (ø)
experimental/ExteriorAlgebra/src/BGG_complex.jl	88.23% <88.23%> (ø)
...xperimental/ExteriorAlgebra/src/ExteriorAlgebra.jl	85.05% <85.47%> (+8.38%)	⬆️

... and 9 files with indirect coverage changes