Fix poorly displayed matrices by replacing text-mode display_matrix
function with math-mode display_matrix_mm function.

These problems used the text-mode display_matrix function in PGmatrixmacros.pl to
display matrices.  \{display_matrix($m)\} inserts very little space between the
columns of $m so many matrices are hard to read.  Replacing it with the
math-mode  \( \{display_matrix_mm($m) \}\) cures the problem.