Handling of physical units

[doomphoenix-qxz]

I see in the core code that there's some use of units from Unitful.jl. While I like the idea of integration with Unitful.jl, I think I would prefer it to be a non-default mode. It could be something like this:

pvtxstate(2.0, missing, 500.0, [0.3,0.4,0.3]) #defaults to bar, m^3/mol, K, but could be easily changed by a function like set_base_units("psi", "ft^3/mol", "R")

Or you could use

physical_units() pvtxstate(2.0u"bar", missing, 500.0u"K", [0.3,0.4, 0.3])

Actually, the physical_units function may be unnecessary--you could just dispatch on whether you receive physical quantities or ordinary numbers as input.

[longemen3000]

i'm gonna check on that, i'm right now fully invested in the equilibrium algorithm, so after having that working, i'm fixing this

[longemen3000]

i implemented a separation in the last commits, the core_(property) just gives the number, where (property) gives the value with units.
i'm thinking to put those functions with units in a separate module, something like this:

SI.pressure(model,v,T,x)

or (the new interfase)

SI.pressure(model,phase)

other options:

• rename to si_property(properties,preferred units)