Suggestions for improvements

[tovrstra]

@dgasmith
I'm not making separate issues for every point below because these are just a bunch of small suggestions that occurred to me when going through the spec.

• Could we just mention on top that atomic units are used throughout? That clarifies some points, e.g. Hartree cannot be the right unit for a frequency.
• results would be a clearer name than variables.
• It is good to have some room for non-standardized results and options. However, just to make this json schema more useful, we'd rather standardize as much as possible, e.g. as follows:
  • Only allow standard keys in the results/variables dictionary. Anything non-standard would better go to a other_results dictionary, such that there are no name collisions later on.
  • Similarly for options used to specify the input: standardize anything that sits in options, while non-standard keys could be put in other_options.
• There seems to be little reason for a separate return_value. It could be included in results/variables.

[tovrstra]

P.S. One more suggestion came to mind: it would be nice to discourage overuse of upper case.