x2fx_names - compound predictor names for x2fx

x2fx() in the statistics toolbox generates additional design variables by multiplying and computing powers of predictor variables passed to it as columns of an input matrix. Unfortunately, if one has NAMES for these predictor variables recorded elsewhere (e.g. {'height', 'speed', 'weight'} there is no way to quickly figure out the names of compound design variables that x2fx() will generate and in what order. This function does just that.