Add explicit storage types for Generator information

[hegner]

As discussed on May 21 provide explicit storage types for MC information.

See issue https://github.com/key4hep/EDM4hep/issues/208

• String values may have to stay as parameters
• Rethink names
• Provide an explicit structure containing the per-event information
• Provide an explicit structure for the ToolInfo
• "Optional" information as much as possible in MCParticle as not to spread information over too many collections. Probably no change to it needed. Color flow etc should be fine there. Optional values can be implemented as storing "0".
• Store kinematics information in double rather than float