update several matrix methods to apply to degree n matrices

[Alex-Jordan]

This builds on #1215. If #1215 is updated following feedback, I will update here too. Once #1215 is merged, this diff should be more manageable.

In addition to changes from #1215, this has:

• some more POD and code comments updated to reflect how methods can be applied to degree n matrices, as well as specify in some places where the method only applies for degree 2 matrices
• make the following methods apply to degree n matrices: isUpperTriangular, isLowerTriangular, isDiagonal, isSymmetric, isOrthogonal, isREF, isRREF. For a degree 1 Matrix, it is interpreted as a 1xk degree 2 Matrix. For degree > 2, these features can apply to all of the degree 2 matrices in the frontal slice.

[drgrice1]

I think this generally looks good. Again some minor code comments.

I tested this by adding some columns to the test problem you gave in #1215, and things are working as expected.

[pstaabp]

This looks good.

[Alex-Jordan]

If we merge this one I'll rebase #1217.