Another master merge into the zoom buffer branch

This was a bit of a messy one.

The changes made to clean up the readability in the regrid and split mean any edits made without those abstractions are a real pig to handle. I think it's worth merging these changes separately (it is easy enough to focus on the uniform box abstractions).

For reasons unknown the sink changes also didn't play nice in the conflicts... that one was more of a mystery to me. Luckily, I know I made no changes there so just took exactly what's on master for those issues.

doc/RTD/source/ExternalPotentials/index.rst

@@ -374,7 +374,7 @@ The parameters of the model are:
       Mdisk_Msun:       6.8e10                 # Mass of the disk (in M_sun)
       Rdisk_kpc:        3.0                    # Effective radius of the disk (in kpc)
       Zdisk_kpc:        0.280                  # Scale-height of the disk (in kpc)
-      amplitude:        1.0                    # Amplitude of the bulge
+      amplitude_Msun_per_kpc3: 1.0e10          # Amplitude of the bulge (in M_sun/kpc^3)
       r_1_kpc:          1.0                    # Reference radius for amplitude of the bulge (in kpc)
       alpha:            1.8                    # Exponent of the power law of the bulge
       r_c_kpc:          1.9                    # Cut-off radius of the bulge (in kpc)