renaming PreImagesRepresentative as NC version

[cdwensley]

PreImagesRepresetnative, PreImages, PreImagesSet and PreImagesElm 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 the package 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 cryst which only uses the first of these 4 operations.

[codecov]

Codecov Report

Merging #31 (4df008f) into master (f1a0f4f) will not change coverage.
The diff coverage is 88.88%.

@@           Coverage Diff           @@
##           master      #31   +/-   ##
=======================================
  Coverage   94.32%   94.32%           
=======================================
  Files          24       24           
  Lines        7573     7573           
=======================================
  Hits         7143     7143           
  Misses        430      430           

Impacted Files	Coverage Δ
gap/pcpgrp.gi	89.51% <60.00%> (ø)
gap/orbstab.gi	86.56% <83.33%> (ø)
gap/cryst.gi	85.62% <100.00%> (ø)
gap/cryst2.gi	86.21% <100.00%> (ø)
gap/equiv.gi	81.41% <100.00%> (ø)
gap/fpgrp.gi	73.10% <100.00%> (ø)
gap/hom.gi	82.14% <100.00%> (ø)
gap/zass.gi	86.88% <100.00%> (ø)

[cdwensley]

Thanks for merging this. Nothing has happened with GAP PR #5073 since October, but your commit helps to encourage me to get back to this development. It will take a long time.