This commit provides a basic frame work for partitioning a frame into
multiple slices, with the continued assumption of single-frame
sequences.

The decoder is modified to independently decode each slice and
accumulate decoded points in a buffer for output.

The encoder is updated to support partitioning the input point cloud
into slices and to independently code each slice.  Points from
reconstructed slices are accumulated and output at the end of the
frame period.

The partitioning process (partitioning methods are defined in
partitioning.cpp) proceeds as follows:

 - quantise the input point cloud without duplicate point removal
   or reordering points.

 - apply the partitioning function to produce a list of tiles
   and slices, each slice having an origin, id, and list of point
   indexes that identify points in the input point cloud.

 - producing a source point cloud for each partition as a subset
   of the input point cloud.

 - compressing each partition (slice) as normal by quantising
   the partitioned input.  Recolouring is necessarily performed
   against the partitioned input since the recolouring method
   cannot correctly handle recolouring a partition from a complete
   point cloud.

NB: this commit does not provide any partitioning methods.