Coordinate system changes are not available

There are no coordinate system conversion functions, such as sph2cart() and cart2sph(). At least sph2cart() is required for the ptsOnSphere(), but it would be a good idea to implement both to allow for conversion from and to Cartesian and from and to Spherical coordinate systems.

It could also be a good idea to do the same for polar coordinates and implement functions for all conversions between the systems.