An internal representation of CLDR data used by this class is a simple
multi dimensional array where keys are nodes' names. If node has attributes,
they are all stored as one string (e.g. 'calendar[@type="gregorian"]' or
'calendar[@type="gregorian"][@alt="proposed-x1001"').
This convenient method extracts a value of desired attribute by its name
(in example above, in order to get the value 'gregorian', 'type' should
be passed as the second parameter to this method).
Note: this method does not validate the input!