delete obsolete comment
Ad-hoc special-case treatment of QFLX in kg/m2/sec.  This may be changed later!
I left in some debugging prints for near-future use in other units work.
Finally, the choice of units to convert to is changed.  It was (when preferred_units
is unset) always the first variable.  Now it's whichever variable has numbers most O(1).
Add another example, some cloud variables.
Fix number_of_diagnostic_plots which no longer was returning a meaningful number.
But also it no longer returns the number of sets of .nc files written out.
Now it's the number of vcanvas2.png() calls.  That's the number of "summary plots".
bugfix: when a variable's units were changed, I had updated varmax but not varmin.
Now varmin is updated too.
Also I left in some debugging code which will be useful in near-future units work.
Merge branch 'devel' of github.com:UV-CDAT/uvcmetrics into devel