MET config modernization and cleanup

This makes some general improvements to the MET config structure, and a few minor C++ updates to tidy up related tools. This should have no effect on any outputs.

• The old-style MET config flags have been completely phased out from the new-style config. Where necessary, they have been replaced by new-style flags.
• Redundant flags to control the usage of PFO links when building MET association maps have been removed/streamlined.
• The ComponentAccumulator-based MET derivation config has been reworked for simplicity and re-named to fit the existing scheme.

Tagging @jcatmore, @williams, @sawyer, @schaarsc for their awareness.