Musicxml

bug fix: ``tree.Tree._reset_iterators()`` has not been added to PyPi package. Via pip installed 'musicscore2' couldn't
work.~~

``XSD_TREE_DICT`` is now used for generating all classes. No performance optimization but a bit tidier.
``__copy__`` added to indicators
``XMLElement.value_`` default value set to '' instead of None
``xmlchildcontainer.requirements_not_fulfilled`` renamed and refactored to ``requirements_fulfilled``
bug fix: if sequene has maxOccur==unbounded parent can be duplicated.

performance optimisation: tree.traverse(), iter_leaves(), reversed_path_to_root() are being cached. To reset call
tree._reset_iterators()
performance optimisation: tree.is_leaf is not checked each time. tree.add_chord() sets parten's is_leaf to False
performance optimisation: xsdcomplextype _XSD_ATTRIBUTES added. This class attribute will be filled only once which
improves XSDComplexType.get_xsd_attributes() method.

!!generating classes still not tested!!

`README.rst`
`intro_musicxml.rst`
`usage.rst`

`XMLElement.xsd_check `attribute added.
`XMLElement.replace_child` returns new child
`XMLSenzaMisura` with default value "" in order to be optional.