Making static gravity more user friendly with custom gravity field flags

[evazlimen]

I have separated out the various hard coded static gravity fields with a switch statement that is determined by parameter file input. This way a user doesn't have to comment and uncomment things in the source code just to run a test.

• added the flag "custom_grav". Used to select the appropriate custom gravity field in static_grav.h. Goes in the parameter file.
• added a switch statment in static_grav.h to use the custom_grav flag. This is set up for the simple integrator, I wasn't sure if it was appropriate to also do so for VL and also wanted to be as minimally invasive as possible! Still have to do some cleaning up but wanted to see if I should be doing this differently.

[evazlimen]

I changed the initialization of custom_grav and set it up for the VL integrator. I formatted with clang-17, not sure if I just have to wait for PR #330 to go through for the formatting check to pass?

[evaneschneider]

Awesome, yep -- we're doing a hack day over here so have a few PRs in the queue waiting on 310 and rebasing, but once #330 330 goes through the clang 17 formatting should work.

[evaneschneider]

FYI, I've pulled in the clang-17 PR, so this one should be good to go once it's updated.