Fix is_isomorphic for matrix group and permutation group

[nathanncohen]

This does not work:

sage: g1 = groups.permutation.PGL(2,2)
sage: g2 = groups.matrix.GO(3,2)
sage: g2.is_isomorphic(g1)
...
AttributeError: 'PGL_with_category' object has no attribute 'gap'

A simple hack seems to work

sage: g1.gap = g1._gap_
sage: g2.is_isomorphic(g1)
True

Though I was not able to find where _gap_ was defined on permutation groups.

Note that g1.is_isomorphic(g2) fails for another reason

sage: g1.is_isomorphic(g2)
...
TypeError: right must be a permutation group

Related:

• Ask Sage question 62674: How to act with PGL on ProjectiveSpace
• Implement matrix representations for finite projective linear groups #12008: Implement matrix representations for finite projective linear groups

CC: @slel @vbraun

Component: group theory

Issue created by migration from https://trac.sagemath.org/ticket/17620