Add normal estimation and region growing algorithm for point cloud
* Add normal estimation and region growing algorithm for point cloud
* 1.Modified documentation for normal estimation;2.Converted curvature in region growing to absolute values;3.Changed the data type of threshold from float to double;4.Fixed some bugs;
* Finished documentation
* Add tests for normal estimation. Test the normal and curvature of each point in the plane and sphere of the point cloud.
* Fix some warnings caused by to small numbers in test
* Change the test to calculate the average difference instead of comparing each normal and curvature
* Fixed the bugs found by testing
* Redesigned the interface and fixed problems:
1. Make the interface compatible with radius search.
2. Make region growing optionally sortable on results.
3. Modified the region growing interface.
4. Format reference.
5. Removed sphere test.
* Fix warnings
* Remove flann dependency
* Move the flann dependency to the corresponding test
