Fix warnings in filters/grid_minimum.h

> grid_minimum.h:83:9: warning: field 'divb_mul_' will be initialized after field 'min_points_per_grid_' [-Wreorder]
Register PointXYZLAB point type properly

This commit adds an auxiliary point type `_PointXYZLAB` (similarly to
how it is done for native PCL point types) in order to fix the invalid
offset warning:

> gicp6d.h:78:1: warning: offset of on non-POD type 'pcl::PointXYZLAB' [-Winvalid-offsetof]
Add a draft of contributing draft
Merge pull request #561 from taketwo/fix-warnings

Fix compiler warnings
Merge pull request #562 from taketwo/add-contributing-guide

Add a contributing guide