black oil fluid system: rename surfaceDensity() to referenceDensity()

because it seems like what eclipse uses in the DENSITY keyword is not
really for surface pressure but at the pressure where the phase's
formation volume factor is one. (this pressure does not seem to be
specified and can potentially be different for every fluid phase.)
Thus rename surfaceDensity() to referenceDensity() and remove the
logic to allow volume formation factors which are not equal to one at
the reference pressure. (i.e., formerly the surface pressure.)