Replace hash_map with unordered_map in 3rdparty/poisson4

[taketwo]

unordered_map was introduced in C++11, so this PR is to be merged after transition to the new standard.

Fixes #2639.

[taketwo]

Rebased on the current master.

[SergioRAgostinho]

The build failed on Mac. These lines looks suspicious because of headers including hash_maps and requiring specific gcc extensions.

https://github.com/PointCloudLibrary/pcl/blob/master/surface/include/pcl/surface/3rdparty/poisson4/multi_grid_octree_data.hpp#L32-L65

[taketwo]

This PR removes these lines, here is how the file looks with it:

https://github.com/PointCloudLibrary/pcl/blob/980825d55be6d43bb8502533768e0f52790fc5fd/surface/include/pcl/surface/3rdparty/poisson4/multi_grid_octree_data.hpp

I suspect these remaining lines are to blame:

pcl/surface/include/pcl/surface/3rdparty/poisson4/multi_grid_octree_data.hpp

Lines 45 to 50 in 980825d

	#ifndef _MSC_VER
	namespace std
	{
	using namespace __gnu_cxx;
	}
	#endif

Will kick them out.