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.
This is difficult. You try to set precision and fixed mode here and undo it later on, but since you don't save the exact state you can't be sure that what you are doing below restores the exact state you had before this line.

I think you either need to find a way to save the complete state of the stream (see for example in deal.II's source/base/timer.cc, in function TimerOutput::print_summary () for an example of how to do this, around line 453 of the current sources) or, maybe simpler, create an ostringstream that you can modify however you like, and then just output the string you just wrote into to pcout.