Minimum refinement function position

This pull request addresses issue #46 and uses the patches suggested in pull request #146. It enables the minimum refinement level function to use not only the depth, but also cartesian or spherical coordinates to prescribe the minimum level of refinement of a mesh cell. It also adds tests for this cases.
Thats ok with me. What shall then happen to the .cc file and the instantiation? Simply remove it / move it to the header?