Use MATLAB builtins “height” and “width”

Since R2013b, MATLAB provides builtins “height” (resp. “width”) for computing
the number of rows (resp. columns) of a matrix.

Octave has been providing “rows” and “columns” for the same purpose for the
long time. For MATLAB compatibility, it introduced “height” and “width” in
version 10, so let’s standardize on MATLAB names.