Map type chunks in general
A mapping of keys (cell values) and values (cell
identifiers) is a general way of storing grid data.
Efficient in terms of memory use where a default
value can be set, and if there are only a small number
of non-default mappings in the chunk (compared to
the number of cells in the chunk)
Offer the means to generating some statistics about a
chunk very efficiently
the diversity (number of different values)