Set the output id (a string; output is a mv) to a format similar to that used
for reduced variables.
- aplusb: call reconcile_units, and add an optional argument to specify the units
- reconcile_units: add an optional argument to specify the common units.  The default
remains to use the units of the first variable.
- new function setunits, returns the variable not the units
class amwg_plot_spec:
  new class member, standard_variables.  This describes reduced variables which may be used by
  any AMWG plot set (i.e., a class derived from amwg_plot_spec).  Presently it has three variables:
  PRECT, AODVIS, TREFHT.  Each of them is sometimes available as a data variable, but sometimes
  has to be computed.

plot set 5:
method _all_variables (hence _list_variables) includes the standard variables in its return value.
  This does _not_ amount to a claim that the variables can be computed, however.
method plan_computation _normal_contours: reorganized so that variables are set up in separate
  functions.  When data is not available for a variable but it is in standard_variables, then
  an attempt will be made to compute it.
  Also, a better job is done of not trying to generate plots which cannot be done.
Add tests for newly implemented diagnostics.  We now can do all 10 Tier 1a plots.
more output for debugging
Merge branch 'devel' of github.com:UV-CDAT/uvcmetrics into devel