Potential bug in test_preparation.py

[nhsavage]

Flake 8 reveals:

./lib/catnip/tests/test_preparation.py:99:9: F841 local variable 'latloncube' is assigned to but never used ./lib/catnip/tests/test_preparation.py:103:13: F841 local variable 'latlon_coord' is assigned to but never used

this may indicate that the test is not working as expected. Tasks:

1. check and fix this error
2. remove other things flagged up F841
3. remove F841 from ignores