Consider adding math only functions for P1D and Px

[andreufont]

It might be good to have math-only functions to compute P1D and Px, somehting that could leave outside of this repo

# define a function that returns the P3D model you want to integrate

# example 1: Arinyo model
def get_func_arinyo_p3d_kt_kp(z, args):
    # if you want Arinyo, sth like this (I made this up)
    p3d_model = forestflow.arinyo.p3d_model(z, args)
    def func(kt, kp):
        return p3d_model.predict_p3d(z, kt, kp, args)
    return func

# example 2: model from McQuinn & White
def get_func_mcw_p3d_kt_kp(args):
    p3d_kaiser = get_kaiser(args)
    def func(kt, kp):
        kp = args['kp_Mpc']
        k2 = kpar**2 + kperp**2
        p3d_suppresion = np.exp(- k2/kp**2)
        return p3d_kaiser.get_p3d(kt, kp) * p3d_suppresion
    return func

# this is the actual code that would be inside the p1d (or px) module
def compute_p1d(p3d_func, kpar, lnkperp=None)
    if lnkperp is None:
        lnkperp = default setup
    kperp = np.exp(lnkperp)
    # integrate p3d_func over kperp
    p3d = p3d_func(kperp, kpar)
    p1d = intengrate_p3d
    return p1d

# and this is how you would use it for Arinyo
ari_p3d_func = get_func_arinyo_p3d_kt_kp(z, args)
ari_p1d = compute_p1d(ari_p3d_func, kpar)

# and for other models
mcw_p3d_func = get_func_mcw_p3d_kt_kp(args)
mcw_p1d = compute_p1d(mcw_p3d_func, kpar)