Generalize the map function for sinks and particles

[nvaytet]

Currently, the map plotting function will only properly handle AMR-type variables (amr, hydro, rt).

It does handle plotting sink particles onto a plane, but in a strange way, with a call to a separate function at the end (_add_scatter), which does things very similar to other parts of map (distance to plane, dot products,...).

It does not handle particles.

We should make the mechanism apply to any type of layer. If the position key is found within the group, use that, if not, go into the amr group.

[nvaytet]

This should now be possible after #132