Make the corner plots use 1, 2, and 3-sigma countours

Based on http://corner.readthedocs.io/en/latest/pages/sigmas.html This makes the countour levels 1, 2, and 3 sigma. Then the quantiles are 1 sigma.

Old behaviour:

New behaviour:

enabled an automatic merge when the pipeline for 2310a8ea succeeds

mentioned in commit 4b558209

merged