Print out a message if can't compute a derived variable.  Usually this is because it depends on
another derived variable, so it will be computed on the second try.
In interp2, don't try to propagate a mask of the wrong dimension, it's probably just False anyway.
Also update doc and comments.
Some changes and reorganization to support AMWG plot set 6 - especially through tuples of plot specs
instead of plot specs.
support tuples of plots (meaning overlay them), in order to support AMWG plot set 6
Compute masks for AMWG plot set 6.
add masks.py for AMWG plot set 6
Extensive changes in AMWG plot set 6.  It now has combined vector-contour plots, and
masks out the continents for CAM files only.
Merge branch 'devel' of github.com:UV-CDAT/uvcmetrics into devel

Conflicts:
	src/python/frontend/diags.py