persist last exposure time between rounds of twiflats

[fergusL]

During twilight flat collection, pocs will loop through all filters specified in config and collect 6-8 flats per filter. If pocs finishes all filters before the twilight window closes, it will start another round however each round will start with the default exposure time for morning or evening twilight flats. This means the first 1 or 2 exposures per round are consistently outside the target count tolerance. "

The take_autoflats method should be adjusted to update a class attribute consisting of a dict of the last exposure time per filter so that each new round can start with a more appropriate exposure time, so that fewer unusable flats are collected.