Add components method to Abelian group

[paemurru]

@simonbrandhorst suggested that instead of accessing the property using the dot notation, a proper getter would be useful. I named the method components since this is how it is referred to in the docs.

[thofma]

Yes and no, see #1466

[simonbrandhorst]

I see. Maybe we just make an arbitrary default decision and go for it? Throw a dice?
We can also add
coefficients(::Type{ZZMatrix}, x)
coefficients(::Type{Vector{ZZRingElem}, x)

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 75.82%. Comparing base (a024dc6) to head (7d4737f).

Files with missing lines	Patch %	Lines
src/GrpAb/Elem.jl	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1632      +/-   ##
==========================================
- Coverage   75.85%   75.82%   -0.04%     
==========================================
  Files         361      361              
  Lines      113702   113704       +2     
==========================================
- Hits        86249    86212      -37     
- Misses      27453    27492      +39     

Files with missing lines	Coverage Δ
src/GrpAb/Elem.jl	95.58% <0.00%> (-1.07%)	⬇️

... and 26 files with indirect coverage changes