Remote neighbour contribution communicates unnecessary zeroed out buffers

[markusbattarbee]

In

vlasiator/vlasovsolver/cpu_trans_map_amr.cpp

Line 1909 in 5e656cf

} else {

we communicate zeroed out buffers in order to handle remote neighbour communication when a lower-refinement level cells is a remote face neighbour. It would be more efficient to instead mark that buffer as unnecessary, not communicate it, and then also on the receiving end not sum it into the already valid data.

This would mean that instead of setting the communicated block count at

vlasiator/vlasovsolver/cpu_trans_map_amr.cpp

Line 1901 in 5e656cf

ccell->neighbor_number_of_blocks.at(sendIndex) = pcell->get_number_of_velocity_blocks(popID);

It'd need to be set inside the following if-region.

I guess then the communication at line 2017 should be split into two; first communicate neighbor block counts, then neighbor block data. Otherwise there should be some failsafe that only the first sibling for each communicate pair gets the blocks.