NC versions of PreImages...

[cdwensley]

PreImages, PreImagesElm, PreImagesSet and PreImagesRepresetnative, can all return incorrect results when the element(s) supplied are not in the range of the map.
This situation has been discussed in GAP issue #4809.
To rectify the situation the plan is to have NC versions of these four operations and to add tests to the non-NC versions.
The procedure to be adopted is as follows.
(1) Rename the four operations by adding 'NC' to their names, and then declare the original operations as synonyms of these. PR #5073 addresses this.
(2) Ask package authors/maintainers to convert all their calls to these functions to the NC versions (unless there is good reason not to do so).
A clause added to init.g ensures that the package still works in older versions of GAP.
(3) Once all the packages have been updated, add tests to the non-NC versions of the operations.
This PR attempts to implement (2) for package FinInG which uses the third and fourth of these operations and installs a method for PreImagesSet.

[codecov]

Codecov Report

Merging #28 (14ab214) into master (4df72a4) will increase coverage by 0.01%.
The diff coverage is 63.63%.

@@            Coverage Diff             @@
##           master      #28      +/-   ##
==========================================
+ Coverage   47.60%   47.62%   +0.01%     
==========================================
  Files          34       34              
  Lines       17920    17926       +6     
==========================================
+ Hits         8531     8537       +6     
  Misses       9389     9389              

Impacted Files	Coverage Δ
lib/gpolygons.gi	24.51% <0.00%> (ø)
lib/orbits-stabilisers.gi	42.58% <0.00%> (ø)
init.g	95.83% <100.00%> (+1.38%)	⬆️
lib/morphisms.gi	21.10% <100.00%> (ø)

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