Add version file compatible with other Key4hep packages

[tmadlener]

BEGINRELEASENOTES

• Add a EDM4hepVersion.h file that has the same basic structure and functionality as other Key4hep packages.

ENDRELEASENOTES

[vvolkl]

@tmadlener Is there a way to write this version to the metadata as well? Would probably need to be done in podio or in the user code?

[tmadlener]

For podio we are already writing the build version of podio. However, there is no "easy" way yet to do it for a generated datamodel like EDM4hep. What could be done at the moment is to write the 64 bit encoded unsigned as user data, or potentially also writing it as 3 ints into the meta data. None of these are ideal, I think and this should be done automatically without having to do it explicitly.

Since writing the version is necessary in any case for schema evolution, this has to be solved in any case rather soon. With that in mind, maybe the current header has to be considered as not completely stable yet.

[tmadlener]

Merging this as it is at the moment, and decide how to deal with the schema version once that is actually available.

[tmadlener]

This does not work when used in the preprocessor. See AIDASoft/podio#374

[tmadlener]

Meeting: Podio could generate this file (and add schema information into it)

[hegner]

I think that would be a slightly bigger thing to discuss. feature creep is a big risk there...