Update to refractometer SG formula

[noone]

It would be nice to have the formula calculating the SG from refractometer reading updated with the one recently proposed by Sean Terrill (http://seanterrill.com/2011/04/07/refractometer-fg-results/), which seems more accurate. Or give the choice to the user as which formula they want to use (old cubic, new cubic or new linear).
I suppose that Brewtarget use the same formula as evryone else, which is the old cubic. It should be fairly easy to change it for the new cubic since the formula is basically the same with only slightly different coefficents.