Add stochtree_version stamp to JSON serialization output

[andrewherren]

Part of #315 (cross-version deserialization, RFC 0002).

Every to_json() call in Python and the equivalent serialization functions in R should write a top-level string field stochtree_version containing the current package version (e.g. "0.4.1").

Python: importlib.metadata.version("stochtree") (fall back to "dev" for editable installs)
R: as.character(utils::packageVersion("stochtree"))

Affects: stochtree/bart.py, stochtree/bcf.py, R/bart.R, R/bcf.R