Draft: Port METNet from 21.2
METNet has long been in need of migration from the 21.2 branch. This finally does that. Outdated setups for accessing information have been updated and evreything has been fit into the modern interfaces.
One substantive change has been made with respect to the 21.2 version: instead of calling buildMETSum(...) to evaluate the final MET, the user calls evaluateNNMET(...) (same signature). This is because buildMETSum(...) was relocated out of the IMETMaker interface a while back since it makes more sense as a standalone function, and what METNet does instead is quite different (and isn't a sum).
Important note: the supported input features are currently unchanged with respect to 21.2. These will be updated, likely in a separate MR, prior to wider deployment.
Keeping as a draft until we've done some basic testing.