EMAN baseline function:
y = a + b*exp( c*sqrt(s) + d*s^2 )
| int n | number of data points. |
| double* x | x array (at least order+1 values). |
| double* y | y array (at least order+1 values). |
| double* coeff | array in which 4 coefficients are returned |
| (if NULL, | no coefficients returned). |