Add Molality units

[peterjamesnugent]

Is your feature request related to a problem? Please describe.
Add Molarity units (e.g. mol/kg).

Describe the solution you'd like
Add mol/kg and other derivative units (e.g. mol/g etc.)

Describe alternatives you've considered
mol/kg are not covered by the existing units (although there are some with molar units).

Additional context
Happy to write the code and raise a PR if you agree it's worth adding.

https://en.wikipedia.org/wiki/Molality#Unit

Not to be confused with Molarity which is per unit volume: https://en.wikipedia.org/wiki/Molar_concentration

[angularsen]

I was convinced this was a typo until you clarified 😄
This seems useful enough, would you be interested in attempting a pull request?

Detailed steps here and I'm happy to help if you have any questions.

https://github.com/angularsen/UnitsNet/wiki/Adding-a-New-Unit