Add units

[ElliottKasoar]

Resolves #131 and #277

• Creates dictionary of units shared by all calculations, to save relevant items when writing out files
• Renames time_fs to time, as this is labelled now by the units.
• Pins default ASE units to 2014, matching current default

One slightly messy aspect is this effectively adds a second, slightly different version of the MD units dictionary. These should probably be combined.

As with all current PRs, this depends on #376 or #377 for tests to pass

[oerc0122]

As always, I approve of adding proper units. One consideration is whether we want to make an issue and add support for pint to handle the units for us and allow users to dump them however they want.

This might be a major piece of work though and again quite pervasive throughout the code. Definitely another PR, but this is a positive change!

[ElliottKasoar]

One slightly messy aspect is this effectively adds a second, slightly different version of the MD units dictionary. These should probably be combined.

The stats units now use the units dictionary in Base, which means they should all be consistent now (e.g. Angstrom was referred to as A in stats, but Ang otherwise).

[oerc0122]

Looks good.

Only other consideration I can think of (having written a unit-handler again recently), do you think Janus will ever need to offer alternative unit schemes or is that someone else's problem?

[ElliottKasoar]

Looks good.

Only other consideration I can think of (having written a unit-handler again recently), do you think Janus will ever need to offer alternative unit schemes or is that someone else's problem?

I can imagine it as an eventuality, but I don't see it happening particularly soon. Perhaps some sort of conversion tool for input/output files would be a first step anyway.

[oerc0122]

Looks good.
Only other consideration I can think of (having written a unit-handler again recently), do you think Janus will ever need to offer alternative unit schemes or is that someone else's problem?

I can imagine it as an eventuality, but I don't see it happening particularly soon. Perhaps some sort of conversion tool for input/output files would be a first step anyway.

That boils down to "someone else's problem" for now. Happy with that.