Add missing include
Multipolygon assembler refactoring
Refactoring of MP Assembler: Make rings and segments into member variables
Move is_below() function out of the class
Extract sub-functions from operator() mega-function in Assembler class
More refactoring of the Assembler class
Pull combine_rings_end/start() functions into Assembler class
Rename and document combine_rings_*() functions
NodeRef ordering
Test and fix calculate_intersection() of NodeRefSegment
Updates to multipolygon code.

Some improvements but still a mess.