Smoothstep implementation is wrong

[nsmith-]

From Thiago, the formula at

rhalphalib/rhalphalib/parameter.py

Line 280 in cdab8c8

formula = "(((0.1875*@0*@0 - 0.625)*@0*@0 + 0.9375)*@0 + 0.5)*TMath::Sign(1, 1+@0)*TMath::Sign(1, 1-@0) + 1 - TMath::Sign(1, 1-@0)"

doesn't behave as it should, blowing up at large values.
I think (((0.1875*x*x - 0.625)*x*x + 0.9375)*x + 0.5)*(x > -1)*(x < 1) + 1*(x >= 1) is what it should be.