Adding separate hist for sumw2

[bryates]

This PR addresses https://github.com/TopEFT/topcoffea/issues/27. Instead of altering how histEFT works, it is simpler to just have a separate histogram for storing the sum of the square of the weights. This will be part of the new_histEFT update, but I wanted a separate PR to make the review easier.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 82.14286% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 26.27%. Comparing base (838b7c9) to head (3f6bff9).

Files	Patch %	Lines
analysis/topeft_run2/analysis_processor.py	50.00%	3 Missing :warning:
topeft/modules/datacard_tools.py	86.66%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## new_histEFT #411 +/- ## =============================================== + Coverage 26.05% 26.27% +0.21% =============================================== Files 31 31 Lines 4532 4552 +20 =============================================== + Hits 1181 1196 +15 - Misses 3351 3356 +5 ``` | [Flag](https://app.codecov.io/gh/TopEFT/topeft/pull/411/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=TopEFT) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/TopEFT/topeft/pull/411/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=TopEFT) | `26.27% <82.14%> (+0.21%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=TopEFT#carryforward-flags-in-the-pull-request-comment) to find out more.

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[Andrew42]

Ok, after the changes you made to my comments, I think I'm good with having this PR merged.