Purpose

New update for the converter consisting of bug fixes and new features.

Features

include xml_snippets in prj-file
also xml-writer for material properties
(both wont be needed in the future since xml-writing will be replaced by coupling to ogs6py, still these function may be of interest, so I suggest to keep them in git history)
get material properties
a function to assign bulk_ids
usage of pathlib for handling paths
a function to assign new material ids to cells that are inactive in feflow