Generate IFOComboMap programmatically
IFOComboMap is currently hard coded for 3 detectors.
It requires 2^nifo entries, so it isn't an obvious problem yet, but it will become hard to maintain.
Also, it's dependent on the ordering of IFOMap, and any change to one requires a change to the other.
We should instead generate it in one go based on IFOMap.
Ensure it's clear from the code/comments that's it's a bitset, i.e. each bit being active corresponds to that IFO being included in the set.
(technically it's a bitset - 1 but that will be fixed in #4 (closed))