Add SmallGroupsAddLayer

[fingolfin]

Closes #67.

Extending the library means allocating two numbers and filling six global arrays. SOTGrps spends 70 lines on it, sglppow the same. This is very brittle, e.g. nothing checks that two packages pick different slots, and load order decides which of two overlapping layers wins.

With this PR, adding a new layer can be done with a simple function call:

SmallGroupsAddLayer( rec(
    name        := "SOTGrps",
    available   := order -> fail-or-record,
    group       := function( order, i, inforec ) ... end,
    id          := ...,          # optional; without it IdGroup stays unavailable
    number      := ...,          # optional; only where `available` omits it
    information := ...,          # optional
    select      := ..., count := ...,   # optional; the generic ones by default
    before := [ ], after := [ "sglppow" ] ) );

name is what other layers refer to a layer by. before and after name the layers this one wants consulted before or after; a name that is not registered is ignored, so wishing about a package that is not loaded does no harm, and wishes that cannot all be met are refused before anything is written. The registered layers are collected in SMALL_GROUPS_LAYERS.

Compatibility

The old route is untouched, so released sglppow and SOTGrps keep working. But new versions of those will make use of this new function, see gap-packages/sotgrps#58 and gap-packages/sglppow#13

For the time being, The eleven layers of this package keep their numbers and are not registered by name. This may or may not change in the future.

Care was taken to test various combinations of old versus new for sglppow/sotgrps/smallgrp, and everything worked fine.

[codecov]

Codecov Report

❌ Patch coverage is 99.07407% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.88%. Comparing base (ed2f323) to head (ac19b7b).

Files with missing lines	Patch %	Lines
gap/addlayer.gi	98.78%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #74      +/-   ##
==========================================
- Coverage   99.88%   99.88%   -0.01%     
==========================================
  Files         509      510       +1     
  Lines      373210   373386     +176     
==========================================
+ Hits       372793   372966     +173     
- Misses        417      420       +3     

Files with missing lines	Coverage Δ
gap/small.gd	100.00% <100.00%> (ø)
gap/small.gi	98.00% <100.00%> (-0.15%)	⬇️
gap/smlinfo.gi	99.05% <100.00%> (+0.01%)	⬆️
gap/addlayer.gi	98.78% <98.78%> (ø)

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