Introduce unified namespaces

This change introduces uniform namespaces

• OpenScenarioEngine::v1_2 (for almost everything...)
• OpenScenarioEngine::v1_2::Node (for e.g. ActivateControllerAction derived from a yase Node)
• testing::OpenScenarioEngine::v1_2 (for testing related stuff, e.g. MockLogger)

Please note that this also changes the namespace for the API Interface from OPENSCENARIO::OpenScenarioEngine to OpenScenarioEngine::v1_2::OpenScenarioEngine

Closes #16 (closed)