Implement StdPresentation

[TWiedemann]

First draft for #338.

Already done

• Recursive construction of StdPresentation for all non-leaf nodes, using StdPresentation of the leaf nodes.
• Tests of the presentation. (Maybe there are some more examples that should be tested.)

Hacks that should be improved

• For the leaf nodes, there is currently only a generic function to construct StdPresentation using IsomorphismFpGroupByGenerators. The "find homomorphism" methods should install StdPresentation when they find a leaf if possible.

Yet to do

• Issue Needed: verification #48

[fingolfin]

Ideally, we'd use InstallMethod(StdPresentation, ..., ri -> CalcStdPresentation(ri)(ri)); then the above line would be redundant

[TWiedemann]

My last commit implements your suggestion and the test succeeds without the redundant line. I am a bit uncertain about some technical details, which we can discuss on Thursday.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.55%. Comparing base (6d84867) to head (dc48298).
⚠️ Report is 19 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #363      +/-   ##
==========================================
- Coverage   78.55%   71.55%   -7.01%     
==========================================
  Files          43       43              
  Lines       18281    18398     +117     
==========================================
- Hits        14361    13164    -1197     
- Misses       3920     5234    +1314     

Files with missing lines	Coverage Δ
gap/base/recognition.gd	100.00% <100.00%> (ø)
gap/base/recognition.gi	70.91% <100.00%> (+3.18%)	⬆️

... and 22 files with indirect coverage changes

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[fingolfin]

Very nice. I am trying to use this to debug issues #343 reported by @ThomasBreuer . Unfortunately it gets stuck in computing a presentation for a lower-triangular matrix group.

In this case:

<recognition node LowerLeftPGroup Size=8388608 Dim=198 Field=2>
``
It is possible (with some work of course) to directly extract a pc presentation for such a group.

I was hoping the `nilmat` package would help, but it doesn't :-(.

However, in fact I am pretty sure I have code for that sitting around somewhere (we have some code in the polycyclic package but unfortunately it seems to only be written with char 0 matrices in mind :-( ).

In the case of the bug, I think that's the only "difficult" node, so if we can fit this in, the mechanism in this PR would help verify that the result is wrong.