openvdb::points::appendGroups 

currently calls

 openvdb::points::appendGroup 

for each group in the supplied vector. This task is to improve the efficiency of appending multiple groups to an OpenVDB Points Tree by only requiring a single threaded pass of the point descriptors and attribute arrays.

Note that only a single unit test for appendGroups exists - it would be worth adding more cases depending on how far the new implementation diverges.