root: add support for ROOT TH2Poly objects for tables with two independent variables

[GraemeWatt]

Related to #37, the ROOT TH2Poly object is a 2D Histogram class allowing to define polygonal bins of arbitrary shape. In the case of a table with two independent variables, where bins of a given independent variable overlap with another bin of the same independent variable, but the 2D polygonal bins do not overlap (a check should be made), TH2Poly histograms could be written instead of the usual TH2F histograms. Here is an example record.

Suggestion from Antoni Aduszkiewicz.