Implement method gap of class PermutationGroupElement properly.

[soehms]

This is a follow up ticket of #26750. In that ticket the method has been implemented as synonym of _gap_, since at that time libgap produced an error for large permutations.

┌────────────────────────────────────────────────────────────────────┐
│ SageMath version 8.5, Release Date: 2018-12-22                     │
│ Using Python 2.7.15. Type "help()" for help.                       │
└────────────────────────────────────────────────────────────────────┘
sage: P = PGU(8,2)
sage: p1, p2 = P.gens()
sage: libgap(p1)
python2: libgap.c:184: libgap_get_input: Assertion `strlen(libGAP_stdin_buffer) < length' failed.
Traceback (most recent call last):
...
ValueError: libGAP: Aborted

This error has been fixed in 8.6.beta0. Therefore, we may have gap using libgap, right now!

Component: group theory

Keywords: libgap, permutation

Author: Sebastian Oehms

Branch/Commit: 0e444c5

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/27234

[soehms]

Branch: u/soehms/permgroup_element_gap_meth

[soehms]

New commits:

0e444c5

27234: initial version

[soehms]

Commit: 0e444c5

[tscrim]

Reviewer: Travis Scrimshaw

[tscrim]

comment:3

LGTM.

[soehms]

comment:4

Thanks!

[vbraun]

Changed branch from u/soehms/permgroup_element_gap_meth to 0e444c5