MATRIX command is now available

[Ben Pfaff]

I just added support for the MATRIX command and the matrix language
that it implements to PSPP. You can find it in the version of PSPP at the
tip of the master branch. It will also be in the next major release.

I've tested each of the commands in the matrix language, but I haven't
started looking for larger examples of its use and trying them out.

I'd appreciate any feedback!