Calculate the Z factor using the Hall-Yarborough method

z.hallyarborough(pres.a, temp.f, gas.sg, n2.frac = 0, co2.frac = 0,
  h2s.frac = 0, as_list = FALSE)

Arguments

pres.a	absolute pressure, psia
temp.f	temperature, deg F
gas.sg	gas specific gravity
n2.frac	N2
co2.frac	CO2
h2s.frac	H2S
as_list	FALSE to return single value z; otherwise returns a list