[WIP] Switch to templated links and remove explicit declarations from yaml file

[tmadlener]

BEGINRELEASENOTES

• Switch to templated podio::Links for the Link datatypes
• Bump the schema version to 3
• Remove the legacy Association headers that were put in place for a smooth transition to Links

ENDRELEASENOTES

This PR introduces all the existing Link datatypes as typedefs (keeping the same header file names for the collections as before) of podio::LinkCollection<ToT, FromT> and removes their declarations from the yaml file. It also includes the necessary schema evolution code for podio to be able to read back data files that have been written with previous versions of EDM4hep (or at least they will once suitable files have been uploaded for testing).

• The typedef headers is in include/edm4hep
• The schema evolution code is in edm4hep/schema_evolution

Both of these can obviously be changed to something more appropriate if necessary.

• Requires Add templated links between arbitrary datatypes AIDASoft/podio#257
• Requires Introduce a new links category for the YAML definitions AIDASoft/podio#691
• Release notes with more information
• Includes Add read tests to ensure backwards compatibility #358
• Updates to the README and documentation for declaring how Links are to be used (covered by podio docs)