Cleanup hom functions for FinGenAbGrp and TorQuadModule

[StevellM]

Needed some cleanup, and there were some problems for trivial groups/empty lists of images.
Deprecate some previous methods which relied on empty lists in input which were not checked; add preferable methods where we enforce an input parent.

@simonbrandhorst

[StevellM]

This should not be merged, yet. I need to fix the deprecations. I just wanted to open the discussion first, see if there were any strong opinions against this change. If not, then I will go through Hecke to fix the deprecations.

[codecov]

Codecov Report

Attention: Patch coverage is 94.93671% with 4 lines in your changes missing coverage. Please review.

Project coverage is 76.47%. Comparing base (1d5878a) to head (6b26e5c).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/AlgAssAbsOrd/PicardGroup.jl	0.00%	1 Missing ⚠️
src/AlgAssRelOrd/NEQ.jl	0.00%	1 Missing ⚠️
src/GrpAb/Map.jl	97.50%	1 Missing ⚠️
src/RCF/rcf_stark.jl	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1835      +/-   ##
==========================================
- Coverage   76.61%   76.47%   -0.14%     
==========================================
  Files         362      362              
  Lines      115012   115035      +23     
==========================================
- Hits        88119    87977     -142     
- Misses      26893    27058     +165     

Files with missing lines	Coverage Δ
src/AlgAssAbsOrd/NEQ.jl	17.68% <100.00%> (ø)
src/Deprecations.jl	100.00% <ø> (ø)
src/GrpAb/Dual.jl	73.91% <100.00%> (ø)
src/NumFieldOrd/NfOrd/RayClassGrp.jl	92.95% <100.00%> (-0.21%)	⬇️
src/QuadForm/Torsion.jl	91.11% <100.00%> (+0.06%)	⬆️
src/RCF/autos.jl	90.47% <100.00%> (+0.01%)	⬆️
src/RCF/class_fields.jl	58.95% <100.00%> (ø)
src/RCF/conductor.jl	78.26% <100.00%> (+0.16%)	⬆️
src/RCF/rcf.jl	90.78% <100.00%> (ø)
src/RCF/rcf_subfield.jl	82.06% <100.00%> (ø)
... and 4 more

... and 28 files with indirect coverage changes

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[simonbrandhorst]

Let me add that the hom(ListOfElements,ListOfElements) and
hom(Domain,ImagesOfGenerators) methods exist only for these types of abelian groups and are not supported in general. I think we should get rid of these methods, because they lead to bugs for corner cases.
Thanks for looking into this Stevell.

[thofma]

I am happy to always have hom(A, B, ...). I can talk to Claus tomorrow morning and see if he has any strong opinion.

[thofma]

No strong opinion from Kaiserslautern.

[simonbrandhorst]

Ideally, we want the types of A and B as stated. The method will complain if there is some type instable or sloppy code and we get a Vector{Any}. These errors can be confusing for beginners.
Do we care? Maybe it would be better to just put A::Vector ? Any opinions?

[thofma]

I agree. Just put Vector

[StevellM]

Sure