FMI2: Parse type and unit definitions into `modelDescription`

[manuelbb-upb]

Based on the changes in https://github.com/ThummeTo/FMICore.jl/pull/33, this pull request modifies parsing of a model definition such that the modelDescription of an FMU2 actually holds non-trivial typeDefinitons and unitDefinitions if present.

There is also post-processing for the model variables to set the attributes according to a declaredType if applicable, i.e., if an attribute is not defined for the model variable but for the relevant SimpleType.

Lastly, there is a convenience function fmi2GetUnit(model_description, model_variable) to retrieve the unit corresponding to model_variable.Real.unit from model_description.unitDefinitions.

Marked as “draft” because:

• I am really new to this whole thing,
• the docs should be adapted,
• maybe these changes should be tested (automatically).

[ThummeTo]

Thank you very much, we will check the PRs and merge if everything is ok. Much appreciated 👍

@JoKircher

[JoKircher]

Sure, I will give it a look

[manuelbb-upb]

Thanks for the review and sorry for not running the unit tests. Will submit a commit soon adding some documentation.