Make the default classes immutable and mark mutable classes explictly

[tmadlener]

BEGINRELEASENOTES

• Make the default classes immutable and mark mutable classes explictly via their class name (e.g. Hit and MutableHit). See a brief discussion in Proposal to make the Const classes default and explicitly mark the mutable ones as mutable #204 for more details on the reasons for this breaking change.
• After these changes collections return mutable objects via their create functionality, and will only give access to the default (immutable) objects when they are const (e.g. when they are read from file).
• In general these changes should make it easier for users to write interface that behave as expected, and also make it very obvious where objects are actually mutated already from looking at an interface definition.

ENDRELEASENOTES

This is a first shot to implement the proposal #204. As a first draft implementation it prepends Mutable to all class names that are mutable and removes the Const prefix from all immutable ones.

Built on top of #143!

• Documentation
• More detailed release notes
• Check how much this breaks EDM4hep and other Key4hep dependencies
• Also rename ConstExtraCode and ExtraCode?

[tmadlener]

Changes to EDM4hep itself amounts to only very few fixes with the current version: https://github.com/tmadlener/EDM4hep/compare/master...tmadlener:mutable-podio-classes?expand=1

[tmadlener]

A more realistic diff for usage where also mutable classes are actually used from k4SimDelphes:

tmadlener/k4SimDelphes@3d8f0f1