Feature/counting algorithms

This feature contains the following modifications:
* Re-factor counting algorithms: `num_points`, `num_segments`, `num_geometries` and `num_interior_rings`: this refactoring involves moving common code to a separate file `algorithms\detail\counting.hpp` and a new namespace that I called `bg::detail::counting`. As part of the re-factoring, the `num_points` code in the `multi/algorithms` directory was also moved in the `algorithms` directory.
* Add variant support for `num_interior_rings` and `num_geometries`.
* Add unit tests for `num_interior_rings` and `num_geometries`.
* Implement `num_points` and `num_segments` for boxes of any dimension: to do this I implemented the Boost.MPL-based metafunction `ipower` in the `bg::util` namespace that computes the power of an integral constant at compile time (there is also a unit test for this metafunction). As part of this change, the unit tests for `num_points` and `num_segments` now test for boxes of dimensions 3, 4 and 5.
open/closed and w/ccw is OK. But we normally construct all of them with the same WKT (which should basically be closed to be valid...) and correct them. Therefore it is basically the same test...
Also because you basically have the knowlegde that num_interior_rings is totally independent from this. Therefore I don't understand the purpose of all these cases (one point more, etc)...