Gplates velocity fix

These are some small improvements plus an important fix for the GPlates plugin. If there was a velocity of 0.0 in the data files, so far an assertion was triggered (because of a normalization ... 0 / 0 is not the normalization of 0), although the calculated value itself was fine.
I think it would be useful to add a comment justifying the value 1e-16 here (it is presumably in m/s?).