Implement SL likelihoods

[slosar]

This is a ticket that is similar to SN likelihood, only that it operates with the time-delay distances, there is no marginalization over the absolute amplitude. Sufficiently different that it needs to be implemented separately, although the implementation can start efficiently as a copy-paste of SN likelihood.
See also ticket: #68

[heatherprince]

I had started working on this, based on Tom Collett's DESC SRD likelihood. I was getting some strange results but haven't looked at in a while, I will dig it up and update here!