Dataset units change and extxyz

[dave452]

Dataset can now read the energies and forces from an extxyz file. Units will be converted when a units specified when writing to a fille.

[ElliottKasoar]

A few things to note (for myself, and to help address the comments):

• When we read a read a file by creating a Dataset we assign units to each frame (frame.units), whether we use ASE or not, so the dictionary of units should always be accessible to change_units_all
• For n2p2 input files, we currently always assume the units are Bohr/Ha, unless each frame has a comment units line
  • I'd like to change that (see: here) but there are a few ways we could address that
• For other input files, we currently always set the units to Ang/eV (ASE defaults), as we do not pass units to the Frames we initialise
  • For these files, units is also a valid argument for ASE's read, but it would never be passed with these changes, as units would not be a kwarg, and so ASE would always assume the default units for the format

My feeling is that as long as we set the Frame units correctly, it shouldn't matter too much if ASE assumes the wrong units. We always call change_units_all when units is passed to dataset.write, so this might actually prevent "double counting".

I might need to expand my changes to allow us to set non-n2p2 input units, but in terms of this PR's changes I'm not sure there should be a problem with regard to reading and writing. We might need to consider if there are occasions where we want to manipulate a Dataset before writing, and if the wrong units would be a problem then.

[ElliottKasoar]

Implemented via #29.