Allow definition of abstract parameters and scenarios

Describe the feature

Scenario parameters can be defined on different levels. According to the VVM project the levels concrete, logical and abstract shall be defined in a machine readable way. Concrete and logical are already covered by OpenSCENARIO 1.1. Abstract means, parameters have no concrete value, no value distribution and optional constraints. Constraints are already possible with 1.1 and value distributions are optional, but the parameters always must have a default value, which makes the parameters concrete and not abstract due to the higher priority of the default value upon execution of the scenario. Therefore at the moment it's not possible to describe abstract parameters.

Describe the solution you would like

The value in the parameter definition shall be optional to allow abstract parameters.

Describe alternatives you have considered

Not support abstract scenarios.

Describe the backwards compatibility

Backward comaptible. Only making a mandatory element optional.

Additional context

My thoughts on the definition of the different levels, examples in M-SDL-like syntax (can be easily transformed to OSC 1.1 syntax), the tranformations between them and execution in a simulation environment are attached here: Parameters_abstract_logical_concrete.pptx