New file clouds.py to support creation of cloud variables.  They are distinguished in that
each variable and its axes have names which depend on the obs you will compare with.
Many new cloud variables in standard_variables.  Their cloud-related axes also are put
in an exclusion list in plot set 5.  And a one-step recursion is added to stdvar2var(),
necessary because of the use of name translations before computations of cloud variables.
Finally, I deleted a few debugging print statements.
Fix nonevals so that a warning message will be less unhelpful.
Move reduce_isccp_prs_tau to amwg/derivations/clouds.py.  Change a "*==None" test to "* is None"
to supress an annoying warning message.  Delete some debugging printouts.
Add a diags runline which I'm currently using for tests.
Merge branch 'devel' of github.com:UV-CDAT/uvcmetrics into devel