refactor to have a geometry type for each feature.

[MFraters]

With @danieldouglas92 adding contours and @Minerallo adding gplates functionality to define the geometry of features, I thought it would be good to have a more generic way of dealing with different geometries.

This solution add the geometry type (e.g. coordinates, contours, gplate) to the name of the feature file (coninental_plate, mantle_layer, etc.) and the same for the class name. We still want the user to be able to chose a model and geometry type separately, also not to break backward compatibility.

I will try it out with a mock contours continental plate, to see if this actually works as expected, but comments are welcome if this is a reasonable approach or not. @danieldouglas92, @Minerallo, @gassmoeller, @tjhei.

[github-actions]

Benchmark	Main	Feature	Difference (99.9% CI)
Slab interpolation simple none	1.141 ± 0.011 (s=410)	1.148 ± 0.011 (s=379)	+0.4% .. +0.8%
Slab interpolation curved simple none	1.130 ± 0.014 (s=409)	1.136 ± 0.010 (s=388)	+0.3% .. +0.8%
Spherical slab interpolation simple none	1.107 ± 0.008 (s=377)	1.111 ± 0.008 (s=437)	+0.3% .. +0.6%
Slab interpolation simple curved CMS	1.180 ± 0.012 (s=378)	1.188 ± 0.012 (s=385)	+0.5% .. +1.0%
Spherical slab interpolation simple CMS	1.455 ± 0.015 (s=317)	1.462 ± 0.013 (s=302)	+0.2% .. +0.7%
Spherical fault interpolation simple none	1.112 ± 0.010 (s=397)	1.118 ± 0.011 (s=412)	+0.3% .. +0.7%
Cartesian min max surface	2.567 ± 0.019 (s=173)	2.586 ± 0.019 (s=179)	+0.5% .. +1.0%
Spherical min max surface	7.268 ± 0.061 (s=61)	7.285 ± 0.087 (s=65)	-0.4% .. +0.8%