Combining alerts from different sources

[martinjohndyer]

At the moment we have new alerts from e.g. Fermi or LVC update themselves as new skymaps come in.

However, it would be nice to be able to update events from different sources. GRBs are a perfect example of this. Take a look at the alerts from 190114C:

The original Swift and Fermi alerts came in within a minute of each other, and then the Fermi final position came ~9 minutes later. In this case we can be pretty certain the event was in tile T0958 thanks to the much better localisation of Swift, interesting that did end up the most likely Fermi tile too but only with 25% probability.

In this case where it's the same type of event and the tiles overlap it's clear what we should have done. Doing that more automatically though will be tricky. Combining coincident GRB and GW alerts will be even more interesting but potentially very useful.

[martinjohndyer]

I should have checked for an existing issue, see #29. Although that's explicitly for GW/GRB and this is Fermi/Swift, the idea is the same.