Add mass-radius curves to population plots

[nichollsh]

Plots mass-radius curves from Zeng+2019 on population plots, so the user is provided with information on how the planet's simulated bulk density compares to different compositions and materials. These data are downloaded from OSF. Closes #216.

Minor things:

• Fixed bug where Tsurf doesns't update when running JANUS with 'fixed' surface condition.
• Fixed typo where density was only being calculated with radius=z_obs, but it should have been radius=z_obs+R_int.
• Clarified that the default case is running without escape enabled.

[nichollsh]

Will convert from draft when examples cases have updated.

[nichollsh]

It's somewhat concerning that the MacOS/Py3.10 test failed at first, and then it passed when I re-ran it. Maybe something we should look into at a later stage. I haven't changed the test data in this PR.

[nichollsh]

@stefsmeets this is ready for review now that the test has passed.