add a preprocessor that casts everything to str

Author: keewis

cf_xarray/units.py

@@ -77,7 +77,12 @@ def repl(m):
     ],
     force_ndarray_like=True,
 )
+# ----- end block copied from metpy
+
+# need to insert to make sure this is the first preprocessor
+units.preprocessors.insert(0, str)
 
+# -----
 units.define("percent = 0.01 = %")
 
 # Define commonly encountered units (both CF and non-CF) not defined by pint