Basic support for cfpint alongside cf_units.

lib/iris/__init__.py

@@ -183,6 +183,7 @@ def __init__(
         date_microseconds=False,
         derived_bounds=False,
         lam_pole_offset=False,
+        use_cfpint_units=False,
     ):
         """Container for run-time options controls.
 
@@ -228,6 +229,11 @@ def __init__(
             to a PP file will set the pole longitude (PP field ``bplon``) to
             180.0 degrees if the grid is defined on a standard pole. Does not
             affect global or rotated-pole domains.
+        use_cfpint_units : bool, default=False
+            When True, units attributes will be created as :class:`cfpint.Unit` by
+            default.  At present you can still assign class:`cf_units.Unit` objects
+            explicitly, and either may be used.  However, support for cf_units will
+            eventually be retired.
 
         """
         # The flag 'example_future_flag' is provided as a reference for the
@@ -243,6 +249,7 @@ def __init__(
         self.__dict__["date_microseconds"] = date_microseconds
         self.__dict__["derived_bounds"] = derived_bounds
         self.__dict__["lam_pole_offset"] = lam_pole_offset
+        self.__dict__["use_cfpint_units"] = use_cfpint_units
 
         # TODO: next major release: set IrisDeprecation to subclass
         #  DeprecationWarning instead of UserWarning.