Calculates the output of Gaussian basis functions for a given set of input
FI = gtm_gbf(MU, sigma, X)
MU - a M-by-L matrix containing the centers of the basis functions
sigma - a scalar giving the standard deviation of the radii-symmetric Gaussian basis functions,
X - the latent variable sample forming the set of inputs; K-by-L
FI - the matrix of basis functions output values; K-by-(M+1), "+1" for a bias basis function
with a fixed value of 1.0