Relax requirements on coal_times

[fritzo]

Addresses #2426
Blocking #2468

This relaxes two restrictions on CoalescentLikelihood that proved inconvenient in #2426:

1. coal_times and leaf_times are no longer required to lie in [0, duration]. Previously the CoalescentTimes* distributions allowed times outside of [0,duration], but CoalescentLikelihood required values in [0,duration]. And it turns out real data shows coal_times before the start of simulation.
2. CoalescentLikelihood no longer requires sorted coal_times. This made sense for the CoalescentTimes* distributions for normalization, but is not needed for the likelihood.

Tested

• added unit tests
• randomly permuted coal_times in an SEIR smoke test