Set the default value of --filterk to 'FILTER'
Set the default value of --gaink to 'GAIN'
Set the default value of --uik to None
Update description of --uik
Set the default value of --margin to zero pixels
Store the path of each imported FITS file only if --uik is given
Update description of --uik
Use more widespread default option values

LEMON was originally designed for its use at 1.23m CAHA, yes, but
there is life beyond this telescope — there is no point in using
default option values such as 'INSFLNAM' (for the FITS keyword that
stores the name of the photometric filter of the observation) when
most of our users will need to change it to the much more common
'FILTER'. Although these new default values are not defined in the
FITS standard, they are certainly at least more widespread.

The most important change is probably that --uik is now disabled by
default (that is, we use to check for saturation the same FITS image
on which we do photometry). It clearly should always have been this
way: --uik is rather easy to overlook and we should not use default
values that may break the application, as happened in issue #23.

This merges branch 'more-common-default-options'