Unify load_xyz and load_xyz_fixed

[jonmarks12]

Currently load_xyz and load_xyz_fixed are separate functions but could easily be combined into one.

[joegomes]

I'll note that the ML-FSM package can be used with periodic boundary condition systems readily, but it required me to implement a similar e.g. load_pdb function to load my system with lattice vectors. It could be good to modify the load function to additionally accept other input file specs. ASE will do this automatically in their load function, but we are currently hard-coding format kwarg. This comes with the caveat that some input file specs don't allow multiple structures per file that we need to read the reactant and product for an initial file, so we should be sure length of the returned Atoms list is correct.