Constants: add hbar and fix (?) some units

[mattwelborn]

Description

This PR adds hbar to constants and fixes some of the units. I don't understand:

1. Were the units "incorrect" intentionally?
2. Why do we have constants separate from the unit registry?
3. Why aren't constants in the unit registry?

Changelog description

hbar was added to constants. The units of c, kb, h, and dipmom_debye2si were corrected.

Status

• Code base linted
• Ready to go

[dgasmith]

See conversation in #198, we should continue this there.

[loriab]

I haven’t put in tests for codata 2018 yet, so any physconst affecting PR should toggle the line at the end of context.py and check that ureg changes are ok for 2018 as well as 2014.

[loriab]

I can't trace back why the units were wrong.

I suspect that most qcel clients are comfortable with direct physical constants and conversion_factor() seems newfangled. So ureg gets most use as user API and isn't so fully built out. I think we want to keep context and ureg separate as they have different sources.

[dgasmith]

To refine, I think we want to keep the PhysicalConstantsContext class, but slowly deprecated the aliases in favor of direct ureg generated ones?

This would be my vote anyways so we have a single source of truth.

[dgasmith]

Merging, moved conversation to #198.