COMP: fix cell2mat bug

This fixes a bug reported by Peter Savadjiev. 

I had made the (erroneous) assumption that all of the elements in the matrix vector had the same number of columns.  This fixes that.

In the way that it is used in fibertractdispersion, this is sufficient. cell2mat is not a complete replacement for the Matlab function, which does ... something when presented with multiple different-sized matrices.