WIP: Turn BaseField into a synonym of BaseDomain (DO NOT MERGE)

[fingolfin]

Note that BaseField was never documented. There are two packages that reference it, though:

1. cvec -- I already addressed that in a commit and will soon make another cvec release with that change in it (it should work with both GAP 4.10 and 4.11 that way)

2. forms (by @jdebeule et al) -- it installs a BaseField method for IsForm objects, which however are not in IsVectorObj nor IsMatrixObj. This worked so far because we did DeclareOperation("BaseField",[IsObject]); but now it doesn't, leading to an error when loading the package. I submitted a PR to make it use InstallOtherMethod here: https://bitbucket.org/jdebeule/forms/pull-requests/1. However, that will only help once forms is released. In the meantime, we need another way to keep forms working. This is the reason why I marked his PR as "do not merge" -- I am not quite sure how to proceed best.

Of course we could just wait till there is a forms release before we apply this PR, and in the meantime, we could apply parts of it (i.e., replace any use of BaseField).

Or perhaps we remove all uses of BaseField, but still leave its BaseField declaration in, and only install a single method which delegates to BaseDomain. That declaration and the single method could go to obsolete.gd/.gi .

Or something else?

Thoughts, comments?

[ThomasBreuer]

As far as I see, also the FinInG package uses BaseField.

Removing the use of BaseField from the GAP library is a good idea.

However, having BaseField as a synonym of BaseDomain is a logical problem when the result is not a field.
(For example, there is an operation FieldOfMatrixGroup that returns Rationals when applied to GL( 2, Integers ) --this result is not useful but at least correct.)