Document the relation between "abelian invariants" and "elementary divisors"

[ThomasBreuer]

• rename the conversion functions between the Vector formats returned by abelian_invariants and elementary_divisors for groups: call them abelian_invariants and elementary_divisors

• document these conversion functions

• make their signatures more restrictive: request a Vector{T} where T <: IntegerUnion instead of just a Vector. (Thus one can no longer enter [] as an argument, for the invariants of the trivial group, Int[] or so is needed.)

This addresses issue #2968.

Alternatively, we could move the definition of abelian_invariants (for FinGenAbGroup) to Hecke.jl, add cross-references between abelian_invariants and elementary_divisors there, and then extend the two functions to GAPGroup in Oscar.jl.

One ugly detail in the current pull request is that referencing abelian_invariants(::Vector) from the docstring of abelian_invariants(G::Union{GAPGroup, FinGenAbGroup}) apparently needs the full signature abelian_invariants(::Vector{S}) where S <: Oscar.IntegerUnion, otherwise I get a wrong circular cross-reference to the docstring itself.
(Referencing elementary_divisors(::Vector) is not a problem, perhaps because elementary_divisors for groups is defined in Hecke.jl.)

[codecov]

Codecov Report

Attention: Patch coverage is 96.42857% with 1 line in your changes missing coverage. Please review.

Project coverage is 84.92%. Comparing base (60ffc34) to head (c124fbc).
Report is 46 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Groups/GrpAb.jl	96.29%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4888      +/-   ##
==========================================
+ Coverage   84.88%   84.92%   +0.03%     
==========================================
  Files         681      685       +4     
  Lines       91621    92183     +562     
==========================================
+ Hits        77772    78282     +510     
- Misses      13849    13901      +52     

Files with missing lines	Coverage Δ
src/Groups/sub.jl	95.72% <100.00%> (ø)
src/Groups/GrpAb.jl	93.43% <96.29%> (+0.31%)	⬆️

... and 72 files with indirect coverage changes

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[lgoettgens]

@ThomasBreuer @fingolfin could you please add appropriate release notes labels to this PR?