aca.acqs has no bad_stars attribute after unpickling

[taldcroft]

This can be worked around by setting aca.acqs.bad_stars = None, but that's probably not a real solution.

In [11]: aca.plot()
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-11-788f6b1375ab> in <module>()
----> 1 aca.plot()

/proj/sot/ska3/flight/lib/python3.6/site-packages/proseco/core.py in plot(self, ax)
    493         if self.acqs:
    494             stars_kwargs['stars'] = self.acqs.stars
--> 495             stars_kwargs['bad_stars'] = self.acqs.bad_stars
    496 
    497         plot_stars(attitude=self.att, catalog=self, ax=ax, **stars_kwargs)

AttributeError: 'AcqTable' object has no attribute 'bad_stars'

[taldcroft]

Re-milestoned to the starcheck release, that's when we'd like this (and in any case there is an easy workaround of getting stars manually instead of having it automatic).