Split radial gravity plugins into separate files

[gassmoeller]

While reviewing #6614 I saw that we pack three gravity plugins into one file. This is not completely unreasonable as they are all quite small. However, I think in line with keeping all plugin files as similar as possible to each other it would be more intuitive to pack one plugin per file as we do almost everywhere else. I split the classes into separate files and deprecated the old header file (it still includes the three new files, so existing code continues to work).

Let me know if the deprecation message using the #pragma message approach is the best way to notify users.

[hyunseong96]

Hi Rene! Was going through #6614 with Hyunseong which led us to this PR and I noticed that this code includes the radial_earth_like plugin. This seems to have been deprecated in the old code and I was wondering if it makes sense to include it here?
Cheers,
Antoniette

[gassmoeller]

Uhm, I am not sure what you mean by if it makes sense to include it here?? I was considering just deleting the radial earth-like plugin, because it has been deprecated for a while and is not a good plugin to use overall (for an earth like gravity profile the ascii data plugin is much better, since it uses PREM as default profile). But then I left it in and treated it the same way as the other radial plugins.

[hyunseong96]

As far as I remember, our intention (@Geoniette and I) was to let you know that there is deprecated class, so you can just remove it without separating it.

[Geoniette]

Hi both! Yep I was just flagging that the radial earth-like plugin was deprecated in the older version but was still included as a plugin in this PR.

[gassmoeller]

Ok, thanks for pointing that out! Actually after your comment I decided to just delete the radial earth-like plugin. I checked and shockingly I disabled that plugin 8 years ago, so it is definitely time for it to go.

This PR should be ready for a review.