Options for numerical mesh selection

[nichollsh]

It would be useful to have different options for setting the height mesh in Aragog.
Possible options:

• Fixed dz throughout the grid
• Finer resolution near edges of the mesh (SPIDER has this already)
• Logarithmic spacing - increasing dz with depth
• Adaptive meshing around the most 'interesting' regions of the mantle (e.g. around the RF).

Related to #36.

[djbower]

Some code already exists in c-SPIDER and could be transferred across to Python (and further improved/generalised).

[lsoucasse]

It seems that Aragog is very close to supporting non uniform meshing as the two key routines that map quantities at basic nodes and spatial gradients at basic nodes from quantities at staggered nodes are written in a general way. It needs to be double checked (as well as boundary condition) and interfaced with the config file accordingly.

[djbower]

Yes, but see also #36. I am in the process of writing up some more extensive notes on mass coordinates, which effectively folds into this question of mesh selection. I will send the notes around once they are more complete.