Improve nice gens functionality and docs

[ssiccha]

So far three commits:

• Turn CalcNiceGens(r,g) into NiceGens(r,g)
  • That is delete the function CalcNiceGens and instead install it as a method for NiceGens with two arguments. Also improves the documentation of NiceGens.
• WIP Remove a line duplicating CalcNiceGensHomNode
  • I need to double-check whether this is correct but the tests look fine.
• WIP Improve documentation of calcnicegens
  • This is still work in progress.

After that I suggest to rename as follows:
NiceGens -> NiceGenerators
calcnicegens -> NiceGeneratorsFunction
CalcNiceGensGeneric -> NiceGeneratorsForLeafRecogNodes
CalcNiceGensHomNode -> NiceGeneratorsForSplitRecogNodes
pregensfac -> PreImagesOfNiceGeneratorsOfImageRecogNode

Here the corresponding input for mine and @FriedrichRober's script so that I don't loose it:

NiceGens,NiceGenerators,Attr
calcnicegens,NiceGeneratorsFunction,Attr
CalcNiceGensGeneric,NiceGeneratorsForLeafRecogNodes,Func
CalcNiceGensHomNode,NiceGeneratorsForSplitRecogNodes,Func
pregensfac,PreImagesOfNiceGeneratorsOfImageRecogNode,Attr

[codecov]

Codecov Report

Merging #278 (c64a1f6) into master (c57a2be) will decrease coverage by 0.00%.
The diff coverage is 90.90%.

❗ Current head c64a1f6 differs from pull request most recent head c125c8c. Consider uploading reports for the commit c125c8c to get more accurate results

@@            Coverage Diff             @@
##           master     #278      +/-   ##
==========================================
- Coverage   77.86%   77.86%   -0.01%     
==========================================
  Files          43       43              
  Lines       18389    18388       -1     
==========================================
- Hits        14319    14318       -1     
  Misses       4070     4070              

Impacted Files	Coverage Δ
gap/base/recognition.gd	100.00% <ø> (ø)
gap/base/recognition.gi	68.79% <88.88%> (ø)
gap/generic/KnownNilpotent.gi	97.18% <100.00%> (ø)

[ssiccha]

It looks like this will become a bigger undertaking. 😅 For now I'll add a lot of experimental code to this PR.

[ssiccha]

When this is done, @aniemeyer would like to have a look at the docs.