Add: load pcl::PolygonMesh from IFS files
Fix: documentation
Update: simplify the API and the code

No deprecated inhere since the methods are private and only used
internally.
Update: clean up around range_grid
Add: ability to load a PolygonMesh from PLY files
Fix: documentation
Merge pull request #356 from nizar-sallem/ifs_support

Add: load pcl::PolygonMesh from IFS files
const correctness fixes