Added becca_plot2() as a generic with two methods: becca_plot2.default (simply a wrapper to the original becca_plot) and becca_plot2.mapvizier(), which takes a mapvizier class object.
becca_plot2.mapvizier() now allows user to write the facetting forumla using the column names of the mapvizier object, rather than using the column names of the used within the becca_plot() funtion that becca_plot2() is serving as a wrapper for.
Added validation to becca_plot2.mapvizier() to check that the subsetting function is included. Updated documentation for becca_plot2() generic.
Changed becca_plot2.mapvizier error message wording for missing subsetting argument.
Merge pull request #25 from chrishaid/becca_plot2

Becca plot2