regrid.py: Always pass a copy to regridder.

Somehow regridder can affect its input! This was manifesting as ds_to_regrid["time"] losing its attributes.

src/landusedata/regrid.py

@@ -58,7 +58,9 @@ def RegridLoop(ds_to_regrid, regridder, test):
 
         # Once the first variable has been included, then we can regrid by variable
         else:
-            ds_regrid[var] = regridder(ds_to_regrid[var])
+            # Somehow regridder() can affect its input in-place! We don't want this,
+            # so we pass it in as a copy.
+            ds_regrid[var] = regridder(ds_to_regrid[var].copy())
 
         # Stop regridding if in test mode
         if test: