code: def SLIT_GAUSSIAN(x,g):
      
      g /= 2
      return sqrt(log(2))/(sqrt(pi)*g)*exp(-log(2)*(x/g)**2)
docs: 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.