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.
What is the usual way we deal with checks for zero? In the gplates plugin I often check for if (abs(x) < eps), but in this case I did not know which eps to take. Does any small number like 1e-14 do?