Rewrite `get_icombo()` to be correct and simpler

get_icombo() currently is unneecessarily complex (since it could just construct the bitset corresponding to each detector in the input) and also lacks any kind of edge case handling.

For example, get_combo("H1H1L1") will give the same result as get_combo("H1L1V1"), because it just looks for the first entry in IFOComboMap that is the same length and contains every detector indicated in the input string.

This could go well with the move to make the IFO combo actually a bitset rather than off-by-one from it.