Fix warnings in common/io.hpp

> io.hpp:275:30: warning: comparison of integers of different signs: 'int' and 'size_type' (aka 'unsigned long') [-Wsign-compare]
Fix warnings in octree/octree_container.h

> octree_container.h:166:7: warning: 'pcl::octree::OctreeContainerEmpty::getSize' hides overloaded virtual function [-Woverloaded-virtual]
> octree_container.h:96:7: note: hidden overloaded virtual function 'pcl::octree::OctreeContainerBase::getSize' declared here: different qualifiers (none vs const)
Fix warnings in sample_consensus/sac_model_circle3d.hpp

> sac_model_circle3d.hpp:283:24: warning: comparison of integers of different signs: 'size_t' (aka 'unsigned long') and 'Index' (aka 'long') [-Wsign-compare]
Fix warnings in sample_consensus/sac_model_registration_2d.hpp

> sac_model_registration_2d.hpp:93:21: warning: unused variable 'pt_tgt' [-Wunused-variable]
Fix warnings in filters/filter_indices.h

> filter_indices.h:186:7: warning: 'pcl::FilterIndices<pcl::PointXYZ>::applyFilter' hides overloaded virtual function [-Woverloaded-virtual]
> filter_indices.h:294:7: warning: 'pcl::FilterIndices<pcl::PCLPointCloud2>::applyFilter' hides overloaded virtual function [-Woverloaded-virtual]
Fix warnings in filters/sampling_surface_normal.hpp

> sampling_surface_normal.hpp:52:30: warning: comparison of integers of different signs: 'unsigned int' and 'int' [-Wsign-compare]
> sampling_surface_normal.hpp:128:13: warning: comparison of integers of different signs: 'const int' and 'unsigned int' [-Wsign-compare]
> sampling_surface_normal.hpp:167:34: warning: comparison of integers of different signs: 'unsigned int' and 'const int' [-Wsign-compare]
Fix warnings in filters/median_filter.hpp

> median_filter.hpp:58:21: warning: comparison of integers of different signs: 'int' and 'uint32_t' (aka 'unsigned int') [-Wsign-compare]
> median_filter.hpp:59:23: warning: comparison of integers of different signs: 'int' and 'uint32_t' (aka 'unsigned int') [-Wsign-compare]
> median_filter.hpp:68:45: warning: comparison of integers of different signs: 'int' and 'uint32_t' (aka 'unsigned int') [-Wsign-compare]
> median_filter.hpp:69:45: warning: comparison of integers of different signs: 'int' and 'uint32_t' (aka 'unsigned int') [-Wsign-compare]
Merge pull request #412 from taketwo/fix-warnings

Fix compiler warnings