Improve (New)ZeroMatrix, (New)IdentityMatrix, (New)ZeroVector to better reject invalid input

[fingolfin]

This is work in progress, but I wanted to have it already visible, so that e.g. @ThomasBreuer has a chance to comment if he wants to. Also, so that I can link to this PR from issues :-).

It should actually be mostly ready, but I need to go over everything once, and there are a few things that should be cleaned up, perhaps a few more tests can be added... We'll see.

[ThomasBreuer]

One aim seems to be to get rid of ConvertToVectorRep calls,
to call the responsible low level functions instead where possible,
and to call CopyToVectorRep (which was available only in HPCGAP up to now) otherwise.

(The same would make sense for ConvertToMatrixRep.)

Concerning the extended argument checks and error messages,
wouldn't it be easier to leave them to CONV_VEC8BIT?

[fingolfin]

This is now ready for review: I've added more tests, dropped some stuff, added missing variants of ZeroVector, and grouped everything into hopefully logical commits which should make the motivation for each change clearer. To further aid with that, I add the tests in an early commit, and then for later commits containing fixes, the effect of the fix is visible in how it changes the test file.

[ThomasBreuer]

I do not want to request changes (thus we can merge this pull request),
but the following thoughts about details should be discussed further.

• Explicitly downranking the BaseDomain method for IsRowVector is motivated by the fact that otherwise the (better) method for IsGF2VectorRep has a lower rank.
  I would argue that the methods for IsGF2VectorRep should better be installed for IsRowVector and IsGF2VectorRep since any object in IsGF2VectorRep will be also in IsRowVector.
  One could define IsGF2Vector:= IsRowVector and IsGF2VectorRep, and use this in the method installations.
  This "concurrence situation" will not occur for other VectorObj implementations, where the defining filter really determines the method that will be used.

• The removal of the "default method" for NewZeroVector is motivated by the fact that constructors cannot have default methods, see the discussion of issue MatrixObj: concerns regarding constructors like NewZeroMatrix, NewVector, etc.  #4398. The documentation which methods are mandatory for a new VectorObj/MatrixObj implementation will depend on the result of this discussion.

[fingolfin]

The main reason I dealt with BaseDomain using -SUM_FLAGS was that the following methods all did the same. But I am happy to have it undone and instead tweak the filters for the BaseDomain methods for Is8BitVectorRep and IsGF2VectorRep as suggested.

However, I wonder why IsRowVector is not automatically implied by IsGF2VectorRep... So I had a look at how we define it:

DeclareRepresentation(
    "IsGF2VectorRep",
    IsDataObjectRep and IsVectorObj, [],
    IsRowVector );

This then lead me to submit issues #4408 and #4409.