WIP: Adding tool to retrieve generator level MET

I added a new tool that adds the generator level MET as a float.

I also figured out/remembered that the generator level particles all have barcodes less than 10000 so I put a mode into the tool that calculates it either way. I double checked by running over 10000 events of one of the filtered samples and the MET values were always exactly equal and bounded by the expected filter values, so we can probably just pick one version and stick with it.

Adding @dguest, @sargyrop and @anghosh for comments.