Create aspect utilities

Created a new file and namespace aspect/utilities for utility functions that are not related to particular plugins or the simulator class. Currently there are two functions implemented, which are conversions between cartesian and spherical coordinates. The conversion from spherical to cartesian coordinates is not used at the moment, because I will need to overhaul the type usage of the GPlates plugin for that at some point anyway, and at that time I will need the cartesian_coordinates function of utilities. Tests that use harmonic perturbation.cc and gplates.cc pass on my machine. For S40RTS we do not have a small test case, maybe @jaustermann could take a look, whether anything changes, but since the other tests pass, I am pretty confident everything is working as intended.
Add to the documentation how the 'dim' components of the returned value have to be interpreted. Like in the following function.