Replace median and quantiles by mode and highest density interval (HDI)

This commit introduces the function 'mode_and_HDI', which computes
the mode (most likely value) and HDI of a distribution. The function
'photometry_collect_results' as well as the templates 'photometry.sl'
and 'kinematics.sl' have been updated to use this function instead
of computing the median and quantiles. While the median and mode as
well as the quantiles and the HDI are identical for Gaussian distri-
butions, they differ for skewed distributions, where the mode and HDI
are better measures of credibility.