[WIP] Make it possible to pass multiple old datamodel definitions to the generator script

[tmadlener]

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Fixes #796

• Includes Generate old POD versions into a version namespace #803
• Includes Refactor schema evolution detection #844
• Includes [WIP] Change the grammar of the schema evolution files #845
• Includes Refactor the way schema evolution tests are done #817
• Actual tests with multiple schema definitions

This PR adds the possibility to pass multiple old datamodel versions to the datmodel generator and adds tests using two old versions. With these changes it should now be possible to pass an arbitrary number of old schemas to the code generator and validate that each evolution from the old version to the current version is possible (otherwise there will be no code generation).

To achieve this the grammar for the evolution files had to be changed to accommodate for the possibility of describing all possible schema jumps in one evolution file.

Compared to #817 the cmake functionality was slightly adapted to accept multiple old datamodel versions and to allow to write to multiple (i.e. exactly two) old versions and reading those files back from one source file.

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