Add single point calculations

[ElliottKasoar]

Resolves #30

Initial code to configure calculators and perform single point calculations for energies, forces and stress.

Largely based on code already in lavello, but refactors, fixes various issues to get tests etc. to pass, and adds more general tests.

Now also adds the ability to return forces and stress.

To do:

• Fix pylint
  • I had previously fixed with with try...except clauses, but it makes it quite verbose. Disabling pylint's complaint is an alternative, since the error message is pretty understandable.
• Add support for trajectory energies
  • The obvious solution is change read to read a specified slice/all structures, and attach the calculator to each image.
  • Are there better ways of doing this? As far as I can tell, calculators can only be attached to Atoms objects.
• Update docstrings in various places
• Add single point forces, stresses etc.
  • How (probably a dictionary)/when do we want to return these?
• Replace UiO66 with smaller structure that we are still able to calculate stress for e.g. NaCl from @alinelena

Note: CI is currently not working due to issues with pulling ASE from GitLab, but all tests pass locally.

[ElliottKasoar]

Current temporary solution to ASE installation is to update the commit once tests start failing, but this may be a problem for publishing etc. if it passes here, but fails when rerun.

[oerc0122]

Not sure if these will come through.