Fix pint-related issues with commenting the pint-xarray lib

In this MR, we simply uncommented the pint-xarray library, allowing this plugin's convert function be used as a dependency for other plugins' DataArray.

src/climpact/convert.py

@@ -6,13 +6,12 @@
 import re
 import warnings
 
-# import cf_xarray.units
+import cf_xarray.units  # noqa: F401
 import pandas as pd
 import pint
-
-# import pint_xarray
-# import xarray as xr
-# from pint_xarray import unit_registry as ureg
+import pint_xarray  # noqa: F401
+import xarray as xr  # noqa: F401
+from pint_xarray import unit_registry as ureg  # noqa: F401
 
 __all__ = ["UnitConverter"]