FirstCohomologyDimension(chr)
chr must be a cohomology-record, created by a call of
CHR(G,p,F,mats). (If there is no finitely presented group F involved,
then the third parameter of CHR should be given as 0.)
FirstCohomologyDimension calculates and returns the dimension
over K = GF(p) of the first cohomology group H^1(G,M) of the group
G in its action on the module M defined by the matrices mats.
GAP 3.4.4