voxel_grid: color fix

[soyersoyer]

downsample the alpha component too

[taketwo]

Hi, could you have a look at a new class introduced in #586? Do you think it may be useful here?

[soyersoyer]

I think it could be useful, except the downsample_all_data_=false case.
Should we create a templatized add function in the CentroidPoint class?

template < typename PointT2 > void add_as(const PointT& p) { ... accumulate as PointT2 ...}


[SergioRAgostinho]

This addresses #1413. Searched the issues put forgot to check pull requests. I also wrote a fix plus a test but only for VoxelGrid. This pull request is missing patching voxel_grid.cpp:177.

I'll try to incorporate #586, this following week.