Isothermal potential

added some more external potentials to gravity, plus a test for energy and angular momentum conservation in the IsothermalPotential/GravityOnly example.

Reassigned to @matthieu

@tt Do we need the file legend.pro for anything ? Seems like a lengthy piece of IDL code.

Added 5 commits:

• 2023020f - Formatting, documentation fixes and dead #define elimination
• 03d6120f - Restore external point mass as default potential
• d1fb9608 - Document the new parameters
• d060ab13 - Updated ICs and run script to match the other examples.
• 38565747 - Make git ignore the files generated by the new example.

Added 1 commit:

• e04174db - Updated ICs and run script to match the other examples.

I have made some style changes to match the rest of the code, updated the documentation and propagated the new parameters where they should be and where they should be documented. @tt are you happy with these ?

Status changed to merged

mentioned in commit b51f933f