Make ghost elements optional

[Amit Patel]

I use ghost elements for my projects but it might be nice if the library didn't assume they were there.

• _r_any_s array should have incoming edges instead of outgoing
• _r_any_s array should have leftmost edge instead of any edge
• r_circulate_* functions need to be updated to use incoming/outgoing instead of outgoing/incoming, and also check for -1
• make ghost step optional in creation api
• have a boolean function to test if a point is on the convex hull, _r_any_s[r] == -1

[Amit Patel]

I believe a0e9604 fixes this