added create() to the fluids class.

Now, instead of:
air = OpenPNM.Fluids.GenericFluid(params)
use:
air = OpenPNM.Fluids.GenericFluid().create(params)

This is so that we have the option to make functions that don't require a fleshed out fluid object to run on. Like this:
OpenPNM.Fluids.GenericFluid().update_in_network(pn,name = 'air')