Speed up FactorGroup and NaturalHomomorphismByNormalSubgroup for pc groups when the normal subgroup lies in an elementary abelian tail of the pcgs

[fingolfin]

When the denominator of a modulo pcgs lies in a tail of the numerator that generates an elementary abelian group, exponents above the tail are those of the numerator, and tail exponents follow by reduction with a row-reduced basis of the denominator. IndexOfElementaryAbelianTail finds the tail, for family pcgs from the collector's avector, and GROUP_BY_PCGS_FINITE_ORDERS skips the trivial relations inside it.

For a group of order 5^2133 modulo a normal subgroup of order 5^787, building the quotient took about 9 hours and now takes 11.5 s.

Assisted-by: Claude Code (Opus 5)

[codecov]

Codecov Report

❌ Patch coverage is 92.92035% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.03%. Comparing base (9ffc3f4) to head (369070f).

Files with missing lines	Patch %	Lines
lib/pcgsmodu.gi	92.85%	5 Missing ⚠️
lib/pcgspcg.gi	89.47%	2 Missing ⚠️
lib/pcgsind.gi	90.90%	1 Missing ⚠️

Additional details and impacted files

@@                            Coverage Diff                            @@
##           mh-claude/pc-normal-closure-normality    #6576      +/-   ##
=========================================================================
+ Coverage                                  79.02%   79.03%   +0.01%     
=========================================================================
  Files                                        683      683              
  Lines                                     295223   295335     +112     
  Branches                                    8637     8661      +24     
=========================================================================
+ Hits                                      233292   233414     +122     
+ Misses                                     60113    60106       -7     
+ Partials                                    1818     1815       -3     

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