Public API to set caldb_root (option 2)

The one where caldb root is a global setting. You can still override it locally by passing caldb_root= when loading constants.

Compared to !1067 (closed), this is simpler, but a bit less flexible.

src/cal_tools/calcat_interface2.py

@@ -214,6 +214,9 @@ def setup_client(
 
 _default_caldb_root = None
 
+def set_default_caldb_root(p: Path):
+    global _default_caldb_root
+    _default_caldb_root = p
 
 def _get_default_caldb_root():
     global _default_caldb_root