ENTROPY // 
FUNCTION: entropy( out, x )
Computes the distribution entropy for each parameter stored in a matrix.

@param {Matrix} out - output matrix
@param {Matrix} x - input matrix
@returns {Matrix} output matrix
function entropy( out, x ) {
	var len = x.length,
		i;
	if ( out.length !== len ) {
		throw new Error( 'entropy()::invalid input arguments. Input and output matrices must be the same length.' );
	}
	for ( i = 0; i < len; i++ ) {
		out.data[ i ] = ENTROPY( x.data[ i ] );
	}
	return out;
}