Use conditional import function to import cartopy in the hrrr_zarr mo…

…dule

herbie/hrrr_zarr.py

@@ -13,11 +13,12 @@
 
 import datetime
 
-import cartopy.crs as ccrs
 import pandas as pd
 import s3fs
 import xarray as xr
 
+from herbie.misc import try_import
+
 
 def load_dataset(urls):
     """
@@ -26,6 +27,8 @@ def load_dataset(urls):
     and latitude/longitude. We also promote the "time" attribute to a coordinate
     so that combining the datasets for each hour will work later on.
     """
+    try_import("cartopy.crs", as_name="ccrs")
+
     projection = ccrs.LambertConformal(
         central_longitude=262.5,
         central_latitude=38.5,