mrmatrix: col/row header handling

[mccalluc]

Pete was considering symmetric matrices, so it actually only keeps the header, I think.

• Not all matrices are symmetric: Consider saving row headers, too.
• Not all matrices have row/column labels: For image raster it's just a distraction. Make optional?

[mccalluc]

#71 handles rectangular matrices without labels, but labelled rectangular matrices are still todo: I propose that it is never plain labels, but instead col_labels and row_labels are always given for labelled data... Alternatives:

• Continue to just use labels if the matrix is square
• Put all of these under a group, to avoid clutter at the top: labels/rows and labels/cols or labels/both.

The work that remains is not a priority for me.