Fix outputs in generated modelDescription.xml

[KristofferSkare]

Description

Resolves #86

Currently the generated FMUs does not pass the fmu checker.
This is now fixed by making two changes to the generation of the modelDescription.xml:

1. Adding the missing tag for the outputs
2. Changing attribute index for to be 1-indexed from being 0-indexed.
3. Change the order of the variables to be added sorted by their valueReference

How Has This Been Tested?

• Generated the example FMUs and checked them using the fmu checker.

Screenshots (if appropriate)

Developer Checklist (before requesting PR review)

• My code follows the style guidelines of this project
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules

I have:

• commented my code, particularly in hard-to-understand areas
• performed a self-review of my own code
• not committed unnecessary formatting changes, thereby occluding the actual changes (e.g. change of tab spacing, eol, etc.)
• made corresponding changes to the documentation
• added change to CHANGELOG.md
• added tests that prove my fix is effective or that my feature works (for core features)

Reviewer checklist

I have:

• performed a self-review of my own code have performed a review of the code
• tested that the software still works as expected
• checked updates to documentation
• checked that the CHANGELOG is updated