clean up

[joachimmetz]

• [x] ~~remove "The default is" in docstrings where not needed~~
• [x] ~~Changes to docstrings and source to match style guide~~
• [x] ~~use path spec factory in tests instead of individual path spec classes - moved to https://github.com/log2timeline/dfvfs/issues/535~~
• [ ] check if _file_object_set_in_init is used, otherwise remove
• [ ] move LOCATION_ROOT to file system base class
• [ ] check and implement GetParentFileEntry
  • implement for tsk_partition_file_entry.py
• [ ] add tests for analyzers
• [ ] change def _Open(self, path_spec=None, mode='rb'): to def _Open(self, path_spec, mode='rb'):