Option to select only events near the Compton cone, for faster processing.

[israelmcmc]

Particularly useful for bright sources (region of interest - ROI). When searching for dim sources, bright sources need to be fully modeled to account for the tails.

[israelmcmc]

Related to #238

I'm working on an EventSelectionInterface in #369. As usual, it'll make this easier to implement this, but well, the specifics still need to be implemented. If someone want to work on this let's coordinate first.