MAINT: Minor tweaks to docstring.
MAINT: In _create_generator only overwrite perf_tracker when it's None.
MAINT: Use filter builtin instead of if-statement and continue.
DOC: Slightly better comment on Frequency._ffill.
DOC: Clarificatory comments in HistoryContainer.
MAINT: Remove unused function in test_rolling_panel.py.
ENH: Adds a new api method `schedule_function`.

schedule_function takes a date rule, a time rule, and a function and
will call the function, passing context and data only when the two rules
fire. This allows for code that is conditional to the datetime of the
algo.

This is implemented internally with `Event` objects which are pairings
of `EventRule`s and callbacks.

handle_data becomes a special event with a rule that always fires. This
makes the logic for handling events more complete and compact.