function getIdentifier( str ) {
	return str.substring( 8, str.length-1 ); // ndindex<XX> => XX
}
Extracts an ndarray index identifier from an ndarray index indexing expression.

@private
@param {string} str - input string
@returns {string} identifier

@example
var str = 'ndindex<0>';

var id = getIdentifier( str );
// returns '0'