Convenience function for loading a part tree. Pickle/JSON is chosen
by trial and error
Parameters :
filename : string
The name of the input file
mode : “pickle” or “json”
Specifies the file format of the input file. The default option is
None, which will make the loader first try to unpickle the file
(which is fast and bug free). Use JSON only if you need to edit
the file in a human-readable way.