Implement access footprint gathering

@jdsteve2 Footprint gathering implemented

python test_statistics.py 'test_gather_access_footprint()'

('a', 'read') [n] -> { n^2 : n > 0 }
('b', 'read') [n] -> { n^2 : n > 0 }
('c', 'write') [n] -> { n^2 : n > 0 }

E.g. this: ``` python test_statistics.py 'test_gather_access_footprint()' ``` says: ``` ('a', 'read') [n] -> { n^2 : n > 0 } ('b', 'read') [n] -> { n^2 : n > 0 } ('c', 'write') [n] -> { n^2 : n > 0 } ```