[filters] Make `UniformSampling` inherit from `FilterIndices` instead of `Filter`

[mvieth]

FilterIndices is a special filter where all subclasses decide for each input point: keep or remove. But no new points are added to the output cloud. UniformSampling does exactly that: it removes some points from the input cloud and keeps other points, but it does not create any new points. So UniformSampling should inherit from FilterIndices. The method applyFilter (PointCloud &output) should be changed to applyFilter (Indices &indices). leaf.second.idx should be added to the output indices, see https://github.com/PointCloudLibrary/pcl/blob/master/filters/include/pcl/filters/impl/uniform_sampling.hpp#L146 See also passthrough.h and passthrough.hpp for another filter that inherits from FilterIndices. Documentation for FilterIndices: https://pointclouds.org/documentation/classpcl_1_1_filter_indices.html