SplitExtension(chr)
chr must be a cohomology-record, created by a call of
CHR(G,p,F,mats), where F is a finitely presented group.
SplitExtension returns a presentation of the split extension of the module
M defined by the matrices mats by the group G.
This is a straightforward calculation, and involves no call of the
external cohomology programs. It is provided here for convenience.
GAP 3.4.4