Harmonize modelDescription.xml

[GallLeo]

Currently, the modelDescription between 2.0 and 3.0 differ more than technically necessary.
They should be as similar as possible, so that users can diff the files and importing problems are limited to differences from 2.0 to 3.0.

Example:
https://github.com/CATIA-Systems/Test-FMUs/blob/master/BouncingBall/FMI2.xml
https://github.com/CATIA-Systems/Test-FMUs/blob/master/BouncingBall/FMI3.xml

Unecessary differences:

• <fmiModelDescription ...> different line breaks
• <UnitDefinitions> missing in 3.0
• <TypeDefinitions> missing in 3.0
• description in <ModelVariables> different descriptions