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

@@ -215,10 +215,16 @@ def setup_client(
 _default_caldb_root = None
 
 def set_default_caldb_root(p: Path):
+    """Override the default root directory for constants in CalCat"""
     global _default_caldb_root
     _default_caldb_root = p
 
-def _get_default_caldb_root():
+def get_default_caldb_root():
+    """Get the root directory for constants in CalCat.
+
+    The default location is different on Maxwell & ONC; this checks which one
+    exists. Calling ``set_default_caldb_root()`` overrides this.
+    """
     global _default_caldb_root
     if _default_caldb_root is None:
         onc_path = Path("/common/cal/caldb_store")
@@ -265,7 +271,7 @@ class SingleConstant:
         if caldb_root is not None:
             caldb_root = Path(caldb_root)
         else:
-            caldb_root = _get_default_caldb_root()
+            caldb_root = get_default_caldb_root()
         return caldb_root / self.path
 
     def dataset_obj(self, caldb_root=None) -> h5py.Dataset: