Support cf_units package if cfunits package is not available (cheyenne)

[hsoh-u]

The cheyenne was configured with cf_units package instead of cfunits. It should use cf_units package if available.

At Cheyenne, cf_units package is loaded with following two commands:
module load python
ncar_pylib

[hsoh-u]

Import cf_units package if cfunits package is not available