Rank down MatrixObj emulation methods

[fingolfin]

Reduce the rank of NumberRows and NumberColumns methods that are meant to
emulate the MatrixObj interface for classic plist-of-list matrices.

These methods were in some cases ranked higher than their counterparts for compressed matrices, leading to unnecessary overhead for e.g. NrRows when invoked on a compressed matrix.

[ThomasBreuer]

For NumberRows and NumberColumns, I agree that these generic methods had a too high rank, and ranking them down is an improvement.
For the other method installations in question, IsPlistRep is explicitly required, thus the methods are not applicable to compressed matrices. What do I misunderstand?

[ThomasBreuer]

The test failures are interesting.

NewVector( IsPlistRep, GF(3), [ Z(3) ] ) is not supported.
With the current pull request, this causes an error when one calls RowsOfMatrix( [ [ Z(3) ] ] ), because the default method for IsMatrixOrMatrixObj calls NewVector in the above form.
Without the current pull request, the RowsOfMatrix method for IsPlistRep matrices is called instead.

One could argue that a NewVector method for the abovementioned situation should be provided such that we can use the IsMatrixOrMatrixObj default for RowsOfMatrix.
On the other hand, NewVector is formally defined only for situations where a vector object is returned, and IsPlistRep objects are not in IsVectorObj.

Thus either the IsPlistRep methods for "higher" operations such as RowsOfMatrix must be ranked above the IsMatrixOrMatrixObj methods, or we need a NewVector method for IsPlistRep, and NewVector must be defined for the more general case that an object either in IsVectorObj or in IsPlistRep is returned.

If we decide that we take the latter point of view then I can create a pull request that provides the missing methods.

[ThomasBreuer]

Fine.
As we had discussed today, we do not require NewVector etc. methods for IsPlistRep matrices.