Date locator for DAILY interval now returns middle of month

The `.matplotlib.dates/AutoDateLocator` has a default of

`interval_multiples=True` that attempts to align ticks with the start of

meaningful intervals like the start of the month, or start of the day, etc.

That lead to approximately 140-day intervals being mapped to the first and 22nd

of the month. This has now been changed so that it chooses the first and

15th of the month, which is probably what most people want.