Instrumental (slit) function.
    B(x) = sqrt(ln(2)/pi)/γ*exp(-ln(2)*(x/γ)**2),
    where γ/2 is a gaussian half-width at half-maximum.

def SLIT_GAUSSIAN(x,g):
    """
    Instrumental (slit) function.
    B(x) = sqrt(ln(2)/pi)/γ*exp(-ln(2)*(x/γ)**2),
    where γ/2 is a gaussian half-width at half-maximum.
    """
    g /= 2
    return sqrt(log(2))/(sqrt(pi)*g)*exp(-log(2)*(x/g)**2)