Rename xyz to position?

[nvaytet]

Currently, the position of amr cells and sink particles are named xyz.
But this is not a very good name, both in the Datagroup and for the figure axes.
(It labels the axes on a map as xyz_x and xyz_y).

• Should we rename this to position to be consistent with the particles position field?
• Another option would be to keep them as separate components x, y, and z instead of making them into a vector, but I don't like this as much as it makes it more difficult to re-center the domain with something like data['amr']['position'] - center (you would now have to have 3 operations instead)

[rteyssier]

I would prefer pos_x for both gas and particle.

[nvaytet]

position for particles just comes from the part_file_descriptor I think?
https://bitbucket.org/rteyssie/ramses/src/b7e8ff7e1c33ca9e52d2c8c59741015c7c6f0890/pm/output_part.f90#lines-76

If you change it there, I will change it here ;-)