MAINT: add event date col field and filter rows where this field is null
TST: modify tests to filter nulls in event date col
MAINT: calculate value repeats by vectorized computation on separate start and end dates.
MAINT: pass DatetimeIndex instead of list of strings
BUG: correctly create asset finder
MAINT: rename fixture
STY: fixes for flake8
STY: add space around assignment
MAINT: add var back to constructor
MAINT: remove unused import
MAINT: compare var with None directly
MAINT: fix merge errors
MAINT: remove record date - not needed.
MAINT: restructure dividends dataset.
MAINT: restructure dividends factors.
WIP: update dividends tests.
MAINT: correct the way to get the 'next' event frame.