Draft: read previous category tree from env

Related to:

• management#821 (closed)
• management#871 (closed)

Reimplement incremental mode reading "PREVIOUS_CATEGORY_TREE" env variable instead of rebuilding category tree only from existing datasets in json-data

.gitignore

@@ -2,3 +2,4 @@
 __pycache__
 .*.swp
 .mypy_cache/
+.venv/