Updates to support HECKformat documents, and minor changes.

- Update copyright.
- Remove manifest.in since we're switching to PDM (and defaults moved
  to the module).
- Remove setup.py since we're switching to PDM.
- Remove chains.yaml, move data to the processor module.
- Fix passthrough in __main__
- Move main function to separate function to support PDM entrypoint.
- metadata.py: Extensive rework
  * Add heck support (lots of little changes to support it). (.heck files can
    replace .meta files)
  * Add yaml metadata support (.meta files can be yaml)
  * Some formatting changes.
  * Make metatree be a little easier to read by separating out
    functionality into extra functions
- processchain.py: Move chains.yaml to a structure internal.
- Add processors/process_heck.py to support the document side of HECKformat
- add pyproject.toml and embrace PDM.

TODO.md

@@ -20,5 +20,6 @@
 * Run commands as part of processing chains
 
 * Project level processing chain overrides in the .meta or whatever.
+* Project settings in separate file from .meta that would basically do .meta stuff. Like global meta + config in a top.heck file by default and overridable by a parameter.