Renaming PreImages... functions as PreImages...NC : GAP PR#5073

[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).
No PR is offered to Semigroups at the moment because it is not clear what to do. In attributes/homomorph.gi there are two methods implemented, and both of these apply tests, so should clearly not be converted to NC. On the other hand, in attributes/isorms.gi there are two new methods implemented which do not do tests, so perhaps should be converted to NC. Apart from these methods, there are just a few calls to PreImagesRepresentative. How would the package authors like to proceed?