Added support for start and end date filters in aggregation class
Merge branch 'rewire_Sepp' of https://github.com/gaberosser/crime-fighter into rewire_Sepp
Fixed sepp tests, now much happier.  May need to test the other types, but at least the basic one is now covered.
STILL TODO: finish shifting plotting outputs from SEPP runner.py to SEPP plotting.py - have already done part of one, but more to go.
Deleted unneeded linkage computing function and removed tests.
Removed need to supply data with SEPP train() call, since data attribute has usually already been set.