Update lens_cosmo.py

[jasonwu224]

No description provided.

[sibirrer]

Thanks for moving the functions to the LensCosmo() class.
Aside from the comments made in the code, can you:

• add test functions for both new routines here: https://github.com/sibirrer/lenstronomy/blob/main/test/test_Cosmo/test_lens_cosmo.py
• can you merge this branch on your side with the branch of your other pull request? It's better to have one single pull request. Every time you update the branch that you made a pull request with, it automatically updates the pull request. In that way we can keep track of the comments and updates and have everything together and I can accept and merge all changes at once.
• add your name and GitHub username to the AUTHORS.rst file

[sibirrer]

This all looks great! I have a minor comment about using pre-computed conversion factors. Let me know if you have troubles with the test functions!