plot_3D_proj: Plots the 3D representation of the projection function
In PaoloLuciano/BPWPM: Bayesian Piecewise Polinomial Model

Given the set of parmeters and the input data in 2D, this function calculates and plots the wireframe on a 3D linear space defined by the input matrix X.

1	plot_3D_proj(X, bpwpm_params, n, f_of_0 = TRUE)

`X`	An input matrix of size n*2.
`bpwpm_params`	an object of the class bpwpm_params of bpwpm_prediction created by the functions `posterior_params` or `predict.bpwpm` respectively that contains all the info about the posterior parametres of the model.
`n`	Thinness of grid for 2D and 3D projectction
`f_of_0`	Logical if If the constant function 0 is to be plotted