Lorentz function used to describe an X-ray diffraction peak
P[0]: Intensity of the peak (maximum intensity of the peak)
P[1]: 2Theta (2 * bragg angle)
P[2]: Integral breadth which is related to the FWHM: IB = pi*FWHM/2 = 1.571*FWHM
func_Lorentz()
easistrain.func_fitting_functions.func_Gauss
easistrain.func_fitting_functions.func_PearsonVII