correct some POD

[Alex-Jordan]

This is phase 1 of my attempt to replace #1076. It seemed to me that some of the POD for lib/Value/Matrix.pm is just incorrect or unclear. So this tries to fix that.

I started thinking of MathObject Matrices as n-dimensional tensors, to help with some confusion when is not a "normal" 2D matrix. I'm using "tensor" some in the POD. There are inconsistencies with how things work when you have a 1D tensor versus nD with n > 1. The inconsistencies would be resolved if 1D tensors were presented as column vectors instead of row vectors. But I suppose we can't make a change like that for backward compatibility reasons, for problems out there that use 1D matrices. With the inconsistencies kept, a lot of the POD is going to need to be like "it works this way for a 1D tensor/matrix but it works this other way for higher D".

Phase 2 will be to rearrange/clean up the POD in the manner of #1076. But that should be easier to review as it will just be moving things around with perhaps some minor rewording.

Phase 3 will be to enhance some things in the existing methods. Specifically I think that row() and column() should be allowed to take multiple index arguments to produce submatrices (e.g. the submatrix of the 1st and 3rd rows).

Phase 4 will be to add @pstaabp's new methods.

Somewhere in there, also add the tests. Maybe some with Phase 3 and some with Phase 4.