Migrate spherical/cartesian geometry conversion steps from aospy_user.calcs.numerics

[spencerahill]

It would make sense for infinite_diff to be able to handle all spherical/cartesian coordinate conversions, rather than having to do them beforehand outside the package.

Perhaps a separate class or module should handle these conversions, however, and infinite_diff call their methods or functions.

[spencerahill]

This functionality now exists within infinite-diff; see advec/phys.py and deriv/phys.py. aospy-obj-lib hasn't been fully migrated over, but that's an issue for that library, not this one.