Allow FITS images with the same date but different filter

Although in theory we can do photometry of simultaneous observations
(FITS files with identical dates but different photometric filters),
we are taking only into consideration the observation date, discarding
those images with the same value even if their filters differ. The fix
is rather easy: consider two images as duplicate only if they have the
same (a) observation date and (b) photometric filter.
Prevent DuplicateImageError
Allow to do photometry on the sources FITS image

Astronomical sources are expected to be detected on a composition of
the FITS images on which we do photometry, in order to maximize the
SNR and allow for a more accurate detection of faint objects. However,
it may be the case that we do not need that much precision, scenario
in which it may be enough for us to detect sources on one of the FITS
images on which we also want to do photometry. In order to make this
possible, we need to ignore the Unix time and photometric filter of
the sources image when one of the images on which we do photometry
has the same values — otherwise, DuplicateImageError is raised.

Kudos to Javier Blasco for the suggestion (#19).

This merges branch 'issue19'