fix typo
Merge branch 'devel' of github.com:UV-CDAT/uvcmetrics into devel
Specify that a plot_set object should have a name and a 'number'.  These are actually specified
in descendant classes.
All plot set classes (descendants of plot_spec) now have a 'number' attribute.
This is a short string containing a number, e.g. '5', or modified number, e.g. '4b'.
This 'number' should be the same was what appears in the 'name' attribute.