GLM: Add unimplemented subcommands, and add a test.

GLM: Add unimplemented subcommands, and add a test.

This change adds a basic GLM test for the "Latin Square" use case.
It also implements the more common subcommands that might occur in
a real life example of such usage.  The /DESIGN subcommand differs
slightly from spss.  In spss, it is optional, and if given can be
empty.  In this change, /DESIGN is mandatory and may not be empty.
The rational for that, is that spss's default is to include interactions
for all the factors.  We don't (yet) support interactions, so it's
better to refuse to run a syntax which relies upon the default
rather than to run it with different semantics.