Add method to clean up xml files.

Journey class can now remove the xml file from the directory.
Serves 2 purposes: avoid cluttering directory/filesystem with
unneccessary files and also avoids issues with ElementTree writing to
same file and becomming un-readable.
GraphBuilder class modified to remove xml files as it goes - comment
out line to see them for debugging purposes.
Use force option when removing xml files.

Avoids writing "file not found" each time the api is called when no
file exists yet.