This commit provides an implementation of m42238 to code occupancy:

 - a node's child occupancy is contextualised based on the adjacent
   neighbours of the parent node.

 - when an isolated node is occupied by only a single child, the
   sub-node address [0,7] is directly coded instead of the 8-bit
   occupancy word.

 - collection of neighbour occupancy is performed using a scatter/
   gather operation upon the insertion of each child node into the
   fifo queue.

 - the feature may be controlled via the command line / config file
   using the --neighbourContextualisation=1|0 option