Alg to record sum of event weights in funny workflows

The sum of the total input events is important to properly estimate simulated backgrounds. Here I'm adding a new algorithms that can save this to several output formats:

• ✅ hdf5 (useful in a lot of our ML workflows)
• ✅ json (just really simple)
• ❌ ROOT (already covered elsewhere)

I skipped ROOT because this is done elsewhere in more standard workflows. I contacted the authors of that code and the recommendation was to keep this separated.