ENTROPY // 
FUNCTION: entropy( out, arr, accessor )
Computes the distribution entropy for parameters stored in an array using an accessor function.

@param {Array|Int8Array|Uint8Array|Uint8ClampedArray|Int16Array|Uint16Array|Int32Array|Uint32Array|Float32Array|Float64Array} out - output array
@param {Array} arr - input array
@param {Function} accessor - accessor function for accessing array values
@returns {Number[]|Int8Array|Uint8Array|Uint8ClampedArray|Int16Array|Uint16Array|Int32Array|Uint32Array|Float32Array|Float64Array} output array
function entropy( out, arr, clbk ) {
	var len = arr.length,
		v, i;
	for ( i = 0; i < len; i++ ) {
		v = clbk( arr[ i ], i );
		if ( typeof v === 'number' ) {
			out[ i ] = ENTROPY( v );
		} else {
			out[ i ] = NaN;
		}
	}
	return out;
}