No longer use VTU files.

VTU files are compressed and so seeing differences in them is impossible. Rather, we should be using a plain text graphical output format such as gnuplot that one can diff and see meaningful differences.
Remove VTU files from converted tests.
Remove one more.
added utilities with cartesian and spherical coordinate conversion

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.

This is a combination of 11 commits by @gassmoeller:
Add utilities.cc and .h
Remove surface_coordinates from harmonic perturbation.
Remove spherical_coordinates function from gplates
Remove spherical_coordinates function from S40RTSPerturbation
Remove unnecessary template argument.
Rename namespace and improve documentation
Simplified calculation and fixed some special cases like dim==2
Added test that explicitly tests the coordinate transformation.
Move implementation of utility functions in header to be inlineable.
Secure check. Remove instantiations. Initialize.
Best practice for dimensionality independence.
fix logic in cartesian_coordinates
Extension of the minimum refinement function.

Add test for minimum refinement function cartesian.

Make sure spherical is not working at the moment.

Implemented spherical coordinates for mesh refinement

Add test for spherical coordinates in minimum refinement function.

Addressing Timos comments
fix one wrong output file from pull request #148
Merge branch 'pr148'
Merge pull request #147 from gassmoeller/minimum_refinement_function_position

Minimum refinement function position
fix test output
fix duplicate header output in output/log.txt

We were writing the welcome message from every processor into output/log.txt. Because the file was overwritten unless we are
resuming from an old computation it was only visible then.