Moved the bulk of AMWG plot set 6, vars_stdvar_normal_contours, to a new class method
stdvar2var().  With this any AMWG plot set can easily compute any standard variable.
1. a bit of reorganization, moved level set calculations to their own method
2. Used standard_variable code now in class amwg_plot_set, to compute
derived variables.  From this we get PRECT, a high priority table entry, for free.
Merge branch 'devel' of github.com:UV-CDAT/uvcmetrics into devel