Fix bugs in NormalizerPcpGroup

[stertooy]

Resolves #122.

It seems there are two separate bugs at play here: one bug affects this function when USE_CANONICAL_PCS@ is true, and another one affects it when the debug variable is false.

In the former case, it seems cc.sol is not always zero, so it must be subtracted.

In the latter case, the assumption in VectorByComplement, i;e. that "igs and fac fit together", is wrong. We can resolve this by calculating a suitable igs ourselves, though this is slower.

I tested this PR with the following code (based on one of the tests in bugfix.tst). It tries 2000 pairs of groups `G

CHECK_NORM@Polycyclic := true;
P2:=SylowSubgroup(GL(IsPermGroup,7,2),2);;
iso := IsomorphismPcpGroup(P2);;
G:=Image(iso);;
for i in [1..2000] do
    Print("RUN ",i,"\n");
    k := Random([2..4]);
    l := Random([1..3]);
    H := SubgroupNC( G, List([1..k],i -> Random(G)));
    V := SubgroupNC( H, List([1..l],j -> Random(H)));
    M := NormalizerPcpGroup( H, V );
    Hpre := PreImage( iso, H );
    Vpre := PreImage( iso, V );
    Mpre := Normalizer( Hpre, Vpre );
    check := Image( iso, Mpre ) = M;
    if not check then
        Error("NORMALISER WRONG");
    fi;
od;

On the current master branch, testing this with USE_CANONICAL_PCS@ set to true usually causes an error within the first 100-ish normalisers. With USE_CANONICAL_PCS@ set to false, usually within the first 20.

With this PR, both true and false passed all 2000 tests.

[fingolfin]

Should get a changelog entry

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.44%. Comparing base (86cd79f) to head (564227e).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #124      +/-   ##
==========================================
+ Coverage   56.39%   56.44%   +0.04%     
==========================================
  Files          87       87              
  Lines       12697    12703       +6     
==========================================
+ Hits         7161     7170       +9     
+ Misses       5536     5533       -3     

Files with missing lines	Coverage Δ
gap/pcpgrp/normcon.gi	85.05% <100.00%> (+0.47%)	⬆️

... and 2 files with indirect coverage changes

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[stertooy]

Changelog entry added, and I improved the tests a bit.

It's going to give a merge conflict with #121 of course, I'll tackle that when it happens :)