Add units commonly encountered in filtration

[nneskildsf]

The process of filtration of a liquid through a filter/membrane is characterized by the flux L/m2/h (imperial gal/d/ft**2) commonly known as LMH (imperial GFD). The flux unit can also be reduced to mm/h which is also somewhat common.

Additionally the flux may depend on the pressure applied which is expressed as [permeability] = [flux] / [pressure] in units L/m2/h/bar.

As such I propose adding:

• LMH
• GFD
• [flux] (or [membrane flux])
• [permeability] (or [membrane permeability]).

It strikes me that these units are relevant for a very specific community of users, and that it might not be popular enough to warrant addition to the default unit registry.
Let me know what you think and if anyone else sees added value.

Br Eskild