FRQ: generalize utils.mesh_to_coords to 3D

[kay-ro]

Proposed new feature or change:

Currently utils.mesh_to_coords() only works with two dimensions. For 3d simulations it would be useful to handle other numbers of dimensions as well.

Here is a first idea of how to generalize the function, where g=[x,y,z] is a multi-dimensional array. It should still be necessary to give x, y and z separately instead of just one array g.

positions = np.array([a.ravel() for a in g]).T