Trailing whitespace.
Implement parse_graphml()

Similar to other formats (e.g., 'parse_gml()'), provide for parsing
graph out of graphml stored in a string (not a file, as is currently
done by 'read_graphml()').
Fix typo in docstring (copy/paste error).
Add tests for parse_graphml() where appropriate.
Merge pull request #840 from cianci/parse_graphml

Implementation of missing function "parse_graphml()"