Review/expand on the use of in_memory for RMSD calculations

[IAlibay]

From Bjarne:

In the MD tutorial's analysis notebook we use:

align_strucs = align.AlignTraj(protein, protein_ref, select="backbone", weights="mass", in_memory=True, verbose=True)

The use of in_memory here is fine as the trajectory is quite small. But if folks copy this to use on a larger system it might cause issues. It would be good to at least have a little bit of a blurb to say that the implications of doing an in_memory transfer.