relax SVN archive

Changed the default save state to XML.

Fixes for load_pickle().

The ds.__reset__() call has been removed because of redundancy and the new ds.is_empty() has been
blacklisted.

Duplicated the test_state() system test into test_state_pickle() and test_state_xml().

This tests both formats.

Added the force flag to the state.load() user function.

When True, this causes the relax data store to be reset prior to state loading.

Updated the relax data store from_xml() method for support of the loading of the XML state file.

Docstring clarification of the dir arg.

Absorbed the load_xml() function into load_state().

After shifting the data store check to load_state() for all formats, the function only had one
statement.

Added a warning to the state.save() user function about the pickle format compatibility.

Added the pickle arg to state.save().

This allows the relax state - the contents of the relax data store - to either be saved as XML or as
a pickled object.

Created the relax data store is_empty() method.