Add forms for Omega(e,d,q), except when d odd, q even; some cleanup

[fingolfin]

Also added a new helper SetInvariantQuadraticFormFromMatrix which reduces some code duplication and as a side effect enforces the invariant B = Q + Q^tr, where B is the Gram matrix of the invariant bilinear form of an orthogonal group G, and Q is the Gram matrix of its invariant quadratic form.

The reason for excluding the case where d is odd, q is even, is described in issue #2576

[codecov]

Codecov Report

Merging #2577 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2577      +/-   ##
==========================================
- Coverage    74.6%    74.6%   -0.01%     
==========================================
  Files         481      481              
  Lines      242383   242354      -29     
==========================================
- Hits       180837   180812      -25     
+ Misses      61546    61542       -4

Impacted Files	Coverage Δ
grp/classic.gi	95.62% <100%> (-0.12%)	⬇️
src/vec8bit.c	83.48% <0%> (+0.1%)	⬆️
hpcgap/lib/hpc/stdtasks.g	64.96% <0%> (+0.25%)	⬆️