[FEAT] Add sky-to-Cartesian coordinate transformation to Python interface

[MikeSWang]

Is the requested feature related to an issue?

No.

Summary

Add sky-to-Cartesian coordinate transformation functions.

Alternatives

N/A

Implementation

Can be implemented in the Python interface through existing astropy dependency.

Additional context

This saves the user from creating separate catalogues with the required Cartesian coordinates.

[MikeSWang]

Rejected proposal

Since the user can easily convert distances using external packages and pass the catalogue as arrays, and we are only able to support standard-model cosmological models through Astropy, this proposal is not impactful enough for additional development time.