Add more `CompatibleVector` variants

[fingolfin]

Right now we have CompatibleVector(M) which takes a matrixobj and returns a "compatible" zero vector.

Here compatible in particular means that the vector has length NrRows(M).

But what if I need a vector for multiplying "from the other side" ? And what if I have an existing vector that I just want to "make ready" for multiplication by M?

So perhaps we should have also at least a function CompatibleVector(v, M) which takes a MatrixObj M and an existing "vector" v (in whatever formats) and which produces a new VectorObj w "compatible" with M such that w * M can be computed.

Then we could also add CompatibleVector(M, v) for which M * w can be computed.

[fingolfin]

Looking at the manual, I just discovered that we have ZeroVector( len, M ) which I guess addresses the need for getting a zero vector that of a length different from NrRows(M) .

[fingolfin]

Another thing that I see again and again: people want a basis for a vector space on which a given matrix or matrix group acts. So if the matrix is A, you might see them do One(A) and then use the rows.

A workaround is to use RowsOfMatrix(OneMutable(A)) but perhaps it would be better to have something like StdBasisVector(A) and even StdBasisVector(A,i) to produce just the i-th std basis vector?

Or maybe I am overthinking it... for now I'll use the RowsOfMatrix approach.