renaming PreImages... functions as PreImages...NC

[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.

No progress has been made on this since February, but now seems a good time to continue with stage (2).
This PR attempts to implement (2) for package Wedderga. This package uses 3 of the 4 functions (not PreImagesSet) and implements new methods for two of them. This PR changes all occurrences to NC, assuming that that is what the authors would wish.

[codecov]

Codecov Report

Merging #98 (a71e678) into master (84e7a6e) will decrease coverage by 0.02%.
Report is 9 commits behind head on master.
The diff coverage is 70.21%.

@@            Coverage Diff             @@
##           master      #98      +/-   ##
==========================================
- Coverage   83.57%   83.55%   -0.02%     
==========================================
  Files          11       11              
  Lines        5193     5193              
==========================================
- Hits         4340     4339       -1     
- Misses        853      854       +1     

Files Changed	Coverage Δ
lib/main.gi	81.48% <55.55%> (ø)
lib/idempot.gi	53.63% <60.00%> (ø)
lib/BW.gi	94.56% <100.00%> (ø)
lib/ExtremeSSPs.gi	100.00% <100.00%> (ø)
lib/crossed.gi	73.91% <100.00%> (ø)
lib/others.gi	76.98% <100.00%> (ø)

... and 1 file with indirect coverage changes

[cdwensley]

We are hoping to get GAP PR#5073 merged soon, even if some packages have not been updated.

[cdwensley]

Many of the PRs for adding the NC versions in various packages have a failing codecov/patch. `I've always assumed that this can be safely ignored, but maybe I'm wrong?

[fingolfin]

@cdwensley it is OK to ignore