CCD.py -- fixed problem with median

Although Window objects are not numpy arrays, somehow the statement np.median(win) is acceptable but it does not return what is expected. This led to a problem that I have fixed, but I still don't know why it did not raise an exception in the first place.
Raw.py -- found and fixed bugs associated with overscan frames

Overscan frames were clearly not consistent with what rtplot was showing. This is now fixed.
gitignore -- added sphinx-related stuff

_build now ignored.