Unwinding of stack, inclusion of a rectangular bounds search, optimization for large numbers of equal points.

[ekovac]

These (incomplete) patches introduce a couple of improvements to jtsiomb/kdtree that wound up being necessary for use in IVPR/Weave.

• Unwound recursion in insert_rec and clear_rec
• Added new recursionless function kd_in_bounds for doing a rectangular search.
• Added an optimization for cases where a point is exactly equal to another point.

Unwinding the recursion is necessary when the number of points is very large. I suggest using the user stack functions I added to also modify the original range query functions, as well as any necessary handling of the equal-point linked list.

[jtsiomb]

Thank you for the contribution! I'll examine the changes as soon as I find some time, and probably merge them with my branch soon.