allow subsets to contain sets themselves

If the number of arguments in subsets is the number of sets (i.e. 2 for
venn2 or 3 for venn3), then assume they are sets and calculate the
intersection areas using set operations.
Cleaned up 'set parameter' fix. Updated tests. Added 'get_circle_center' and 'get_circle_radius' to Venn2 and Venn3 objects
Version 0.7