Break up statistics.py into pieces

[uchchwhash]

The datacube_stats.statistics module contains quite a few statistics classes. And the list is growing.

We want to keep relevant statistics together in separate groups.

• Replace datacube_stats/statistics.py file with a module containing several sub-modules
• The top level module still looks pretty much the same to keep backwards compatibility
• Add mangrove_canopy_cover to the list of available stats
• Remove half-hearted automatic discovery code in setup.py