Added "from_r0" to Burkert potential

[HSouch]

Added a method to generate the Burkert potential from just the r0

This is standard practice, as it is ultimately a 1-parameter potential. The relationship between density and core radius is described in Equations 4 and 5 of Mori & Burkert 2000.

Checklist

• Did you add tests?
• Did you add documentation for your changes?
• Did you reference any relevant issues?
• Did you add a changelog entry? (see CHANGES.rst)
• Are the CI tests passing?
• Is the milestone set?

[adrn]

Sorry for the long delay! I rebased your PR, so pull locally before you make any additional changes. I left one suggested change and one comment on the implementation. Thanks!

[adrn]

Thanks @HSouch!