Generalised CoM to take lists of 2D arrays

The calculate_center_of_mass function, which I previously sped up, only took individual 2D arrays, and returned tuples of center positions. I've generalised this to "nD-2D" arrays, that is, array inputs of shape (N1, N2, ..., ND, Y, X) will now return center position arrays of shape (N1, N2, ..., ND, 2). The behaviour for regular 2D arrays of shape (Y, X) is unchanged.

I'm intending to use this for my own work outside of Atomap, but I thought it would be nice to share it. It should be useful in the atomap library as well, e.g. feeding an array of atoms to it.