Negative invB(b) factors #514

Due to linear extrapolation we can get negative invB (b) factors for some setup of the PVT tables when computing well potentials for default pressure (i.e. 1 atm)

PVDG
25 0.05
50 0.02
..
/

should give b approx 0.06 and invB = 1 / 0.08 >0 for 1 atm.

but since we instead extrapolate invB we get the following

25 20
50 50
..
/

which gives approximately -10 for invB for 1 atm.

@bska @OPMUSER Any suggestions on how this should be fixed.
I have some like adding an extra data point for 1 atm (or 1 bar) in the invB table where values are computed based on extrapolation of the input table or change to a constant extrapolation of the table. But comments are very welcome.