add ConformalSymplecticGroup and some generalizations

[ThomasBreuer]

Add ConformalSymplecticGroup (addresses #43),
essentially analogous to the implementation of SymplecticGroup:

• introduce a plain function ConformalSymplecticGroup and a constructor ConformalSymplecticGroupCons,
• install various methods for ConformalSymplecticGroupCons,
• introduce a property IsFullSubgroupGLRespectingBilinearFormUpToScalars that is set in the groups returned by ConformalSymplecticGroup,
• install a \in method for a matrix and a group in IsFullSubgroupGLRespectingBilinearFormUpToScalars,
• add tests for the new code.

The proposed implementation differs from that of SymplecticGroup and is intended as a pattern for changing the implementation of SymplecticGroup and the other classical groups, and for adding the other types of conformal groups.

These are the differences:

• Do not delegate from the variants where a finite field is given to the variants with given size of the field, but the other way round. This way, it will be possible to use elements from StandardFiniteField( p, n ) instead of GF( p, n ) in the matrices.

• We might recommend to choose an IsMatrixObj representation for the matrices if the entries are from StandardFiniteField( p, n ).

  For that (and also in general), an "experimental" global option ConstructingFilter is supported that prescribes the internal format of the matrices. Currently one can use for example IsPlistRepMatrix as the value for this option, such that one can make experiments with matrix groups whose elements are in IsMatrixObj. More useful IsMatrixObj representations will hopefully become available. (Note: We can view IsPlistRep as the default for the ConstructingFilter option, but then the matrices need not be in IsPlistRep because the ImmutableMatrix calls in the code convert the generators to IsGF2MatrixRep or Is8BitMatrixRep if possible.)

Once we decide about the setup that we want for ConformalSymplecticGroup, the implementation of SymplecticGroup etc. in the GAP library and the relevant methods in the Forms package can be rewritten accordingly.

[codecov]

Codecov Report

❌ Patch coverage is 93.59431% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.08%. Comparing base (d876d7a) to head (602c828).

Files with missing lines	Patch %	Lines
lib/conformal.gi	93.20%	18 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #80      +/-   ##
==========================================
- Coverage   81.05%   80.08%   -0.97%     
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  Files           6        8       +2     
  Lines        3663     3786     +123     
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+ Hits         2969     3032      +63     
- Misses        694      754      +60     

Files with missing lines	Coverage Δ
lib/conformal.gd	100.00% <100.00%> (ø)
lib/conformal.gi	93.20% <93.20%> (ø)

... and 4 files with indirect coverage changes

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[ThomasBreuer]

Apparently we have the problem that the current matrix object machinery from GAP cannot be assumed because the Forms package wants to support also the GAP versions 4.9 to 4.12, which do not provide all functions needed here.

The proposed code cannot be added to the GAP library because part of it deals with bilinear forms which are provided by the Forms package, and if we want to support matrix objects for the group generators then we have to support them also for the matrices of the forms, otherwise we would not be able to multiply a group element with a matrix of a form.

It would be tempting to regard the proposed code as a package extension, depending on GAP 4.13.0 or newer; logically, this would fit. Unfortunately, it does not work because dependencies on the GAP version are not supported (yet).

Pragmatically, we can also argue that the code would better be added to the Recog package, which is free to require good enough versions of GAP and of the Forms package.