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.
@barendgehrels Regarding your comment about `num_points` (and that it always returns 4): the default value for `add_for_open` is `false`, which means that for an open triangle you would get 3 by default, and 4 only if you set `add_for_open` to `true`. So you do not **always** get 4...

Besides the above remark, stated only for completeness and nothing more, I do see your point, and I agree: there is no need for the boolean parameter. I will remove it, and adjust the implementation accordingly.