Update your PYTHONPATH#
Make sure that the parent directory of cassis_lte_python (called cassis-lte-python unless you renamed it) is in your PYTHONPATH.
You can do so temporarily within python :
import sys
sys.path.append('/path/to/the/directory/cassis-lte-python')
or more “permanently” :
in your .bashrc :
export PYTHONPATH="${PYTHONPATH}:/path/to/the/directory/cassis-lte-python"
in your .cshrc or .tcshrc :
setenv PYTHONPATH $PYTHONPATH:/path/to/the/directory/cassis-lte-python