Expand documentation on simulation tools

[WardLT]

There are many subtleties to how ExaMol uses our simulation codes. We should expand the documentation page to explain what they are and why they were chosen.

A few things I know we need

• CP2K only exposes a limited set of XC functionals, because we must optimize a cutoff energy for each
• Some of the CP2K methods assume nonperiodic whereas other require periodic cells
• Gaussian uses its own optimizer for small cells but ASE's for large ones due to issues in ASE's parsers