Can't use underscores in names of producers

[bradenpecora]

Currently, the type of a producer can't contain underscores. It won't raise any errors, but things will not be plotted correctly as a result of the string matching used. Specifically, here is why: https://github.com/bradenpecora/HOwDI/blob/8392fb5cd335d43f96a51522c907dab4c050c06c/HOwDI/postprocessing/generate_outputs.py#L71-L76

If a producer is named/named before_after, the corresponding key in the returned dict will be before. It should be before_after.

I don't intend to fix this anytime soon, but someone can make a PR if they like. There should be a warning at the very least.