Add metadata to the ChangeSet

[ewuerger]

In order to know which model the calculated ChangeSet is for, which Capella version , which capellambse version and what revision, revision hash and branch (if versioned model) was used, this information should be included into the ChangeSet.

It should look like the following:

capellambse: 0.5.15
model:
  url: git@ssh.xyz/example.git
  version: 12345678...
referencing: implicit
generator: Capella-RM-Bridge v1.0.0
---
...

referencing will inform about the type of the ChangeSet. If referencing: explicit is set it is expected that the ChangeSet includes all implicit changes as well (actions to references).