Move creation and initialization of canvases out of the loops.  It only has to be done once, and
the 'LINE' stuff will produce an error if you try to do it more than once.
Because of this change, I also had to put in a vcanvas.clear() line.
1. reduce2latlon_seasonal lets you exclude axes from the reduction - needed for cloud calculations
2. new function reduce_isccp_prs_taul for cloud calculations
3. reconcile_units understands 'fraction'
Support for cloud variables, mainly in the standard_variables list.  More will
have to be done, but this is a useful start.