Allow function-call syntax for general mappings.

[stevelinton]

#2006 Description
Inspired by a recent GAP Forum request. This PR allows a function-call like syntax for taking images of general mappings. Specifically map( x ) is treated as Image( map, x) whenever map is a general mapping.

It's really just for comments. The change is just a couple of lines, plus tests and a note in the manual,
so it seemed better to demonstrate the idea before discussing, but I'd not wedded to it.

Text for release notes

Allows a function-call like syntax for taking images of general mappings.

Incorporates #3902 which should be addressed first.

If this pull request should be mentioned in the release notes,
please provide a short description matching the style of the GAP
CHANGES.md file in the root directory.

Further details

If necessary, please provide further details here.

Checklist for pull request reviewers

• [x ] proper formatting

If your code contains kernel C code, run clang-format on it; the
simplest way is to use git clang-format, e.g. like this (don't
forget to commit the resulting changes):

git clang-format $(git merge-base HEAD master)

• usage of relevant labels

  1. either release notes: not needed or release notes: to be added
  2. at least one of the labels bug or enhancement or new feature
  3. for changes meant to be backported to stable-4.X add the backport-to-4.X label
  4. consider adding any of the labels build system, documentation, kernel, library, tests

• runnable tests

• lines changed in commits are sufficiently covered by the tests

• adequate pull request title

• well formulated text for release notes

• relevant documentation updates

• sensible comments in the code

[ChrisJefferson]

This feels like an improvement to me. If we decided we really didn't want this, I'd still want to add a function which told people to use Image.

[fingolfin]

Many people have been confused by this in the past, and I got a question about this just a few weeks ago, so...

The one objection I can think of is that GAP actions are always right actions, and this syntax is a strictly speaking a syntax for left actions; i.e., we have x^(f*g) = (x^f)^g but (f*g)(x) = g(f(x)). But I think that's not a major issue; though it might be worthwhile to point this property out explicitly in the documentation.

Please rebase so that tests have a chance of running.

[stevelinton]

@fingolfin Agreed about a warning in the documentation -- added "do not merge" until I have a minute to add it

[coveralls]

Coverage remained the same at 84.779% when pulling 8d29de9 on func-auts into 9f22a72 on master.

[stevelinton]

Documentation warning added.