Added Example Custom Model/Constraint Functions

import numpy as np

def DeltaCustom(parameters, parameter_names, x, resolution, Q):

    """Calculate the Elastic line for values within +/- 100 of the peak maximum.

        Parameters:

        0: Center

        1: Amplitude

    """

    xx0 = parameters[parameter_names[0]].value

    AMP = parameters[parameter_names[1]].value

    delta=np.zeros(len(x))

    jlower = 0

    jupper = len(x) - 1

    while (jupper - jlower > 1 ):

        jmedium = (jupper+jlower) >> 1 

        if ( xx0 >= x[jmedium]):

            jlower = jmedium

        else:

            jupper = jmedium

    delta[jupper] = (xx0 - x[jlower])/(x[jupper] - x[jlower])        

    delta[jlower] = (x[jupper] - xx0)/(x[jupper] - x[jlower]) 

    delta[jupper] = delta[jupper] * AMP / (x[jupper] - x[jlower])

    delta[jlower] = delta[jlower] * AMP / (x[jupper] - x[jlower])

    """

    idx = (np.abs(x-xx0)).argmin()

    pirulo=(x[idx]-xx0)/(x[idx+1]-x[idx])

    if pirulo>0:

        delta[idx-1]=pirulo

        delta[idx]=1-pirulo

    else:

        delta[idx]=1+pirulo

        delta[idx+1]=-pirulo        

    """

    return delta

def get_parameters():

    """Returns the string description of the parameters for this function. The strings start with the parameter name, then

    followed by a comma are keyword arguments in the form "keyword=value", separated by commas. This function must return at

    least one keyword argument in the string, "value", to set the initial value. Example: "center,value=0.5"

    """

    return "cen,value=0.0,min=-3900,max=3900 amp,value=1.0,min=0.0,vary=True"

def FirstOrderSphericalBessel(symtable, X, sin, cos):

    """Performs the first order spherical bessel function on the given variable.

    """

    return (sin(X) / (X**2)) - (cos(X) / X)