Add tests for NMO

[fingolfin]

e.g. by adapting the examples in the NMO manual. Or perhaps we can just use what's in doc/examples/nmo and treat it like the examples in doc/examples. That will require understanding and modifying the scripts taking care of that right now. Alternatively, the whole code dealing with that could be rewritten to make use of the AutoDoc / GAPDoc "extract_examples" feature. But beware, the NMO manual is not (yet) regenerated using AutoDoc.

[cdwensley]

Tests from the manual added by PR #30 in gbnp/tst/nmo.tst. (Manual to follow.)