FITSImage.date(): fix error in the format passed to time.strptime()

The regular expression that attempts to match the old date format,
'yy/mm/dd', uses dashes instead of slashes (i.e., it tries to match
something different, 'yy-mm-dd'). No wonder why the method, we just
found out, was not working with FITS images that use this format.
Merge two of the regular expressions used by FITSImage.date()

According to the FITS standard, the new Y2K compliant date format is
'yyyy-mm-dd' or 'yyyy-mm-ddTHH:MM:SS[.sss]. Although we wrote this as
two different regular expressions, it is not very difficult to notice
that these two formats are equivalent to yyyy-mm-dd[THH:MM:SS[.sss]].
We can also use group names to extract the matched substrings with
more clarity.
Fix typographical error — it's "no matter what", not 'that'
Avoid empty subdirectories of automatically discarded FITS images

Postpone the creation of the subdirectories where images that are
automatically discarded are saved until we can confirm that at least
one image is effectively going to be so. In this manner, we avoid
ending up with empty directories if for any of the criteria (FWHM
and elongation) nothing is discarded.
Fix typographical error — 'inored' should read 'ignored'
Iterate over the FITS images in order as they are discarded