Adjust code to match current FLANN API

With FLANN 1.8 they switched from int to size_t. PCL is still using int, so there is always an implicit conversion.

I tried to fix it in context of CLang-Tidy warning performance-implicit-conversion-in-loop here:
https://github.com/PointCloudLibrary/pcl/blob/816391762918f3a026b1822fe42c74bb83129eb4/registration/include/pcl/registration/impl/ppf_registration.hpp#L108

But after this I had to change a lot of further PCL code, so I stopped my work after I had already ~100 lines adjusted, because I don't know if you want such a big change before release of 1.10.

Necessary adjustments:

• nearestKSearch
  https://github.com/PointCloudLibrary/pcl/blob/816391762918f3a026b1822fe42c74bb83129eb4/kdtree/include/pcl/kdtree/kdtree_flann.h#L160-L162
  =>

 size_t  
 nearestKSearch (const PointT &point, size_t k,  
                 std::vector<size_t> &k_indices, std::vector<float> &k_sqr_distances) const override;

• radiusSearch
  https://github.com/PointCloudLibrary/pcl/blob/816391762918f3a026b1822fe42c74bb83129eb4/kdtree/include/pcl/kdtree/kdtree_flann.h#L180-L182
  =>

int 
      radiusSearch (const PointT &point, double radius, std::vector<size_t> &k_indices,
                    std::vector<float> &k_sqr_distances, int max_nn = 0) const override;

Some things you should notice about this:

• max_nn is an int instead of unsigned int. We should think if we adjust meaning of 0, because -1 means to return all points in FLANN during I don't know what FLANN is doing with 0.
• radiusSearch returns int during nearestKSearch returns size_t, as nearestKSearch just return k. Maybe we should not return k, but return value of knnSearch.

And in general: Increase required FLANN version in CMake from 1.7 to 1.8.

Start with kdtree/include/pcl/kdtree/kdtree_flann.h and the ppf_registration.hpp loop, then inspect the related PCL call sites affected by FLANN 1.8's size_t API. Decide the intended max_nn and search-result semantics, update the CMake minimum FLANN version, and confirm all affected declarations and uses are consistent.