requirements.txt: added some necessary modules for quickstart.

requirements.txt

@@ -2,15 +2,17 @@ numpy
 matplotlib
 netcdf4
 xarray
-#zarr
+#zarr            # cannot easily be installed with pip
 pandas
 cmocean
 scipy
 ipdb
 ipywidgets
+ipykernel
 jupytext
+nbconvert
 pytest
 healpy
 cftime
-#cartopy
-#dask[complete]
+#cartopy         # cannot easily be installed with pip
+#dask[complete]  # cannot easily be installed with pip