Made a bunch of change to Model, adding output and replace old property construction with new property construction

clean up old convention in Model

Fix a bunch of bugs, take care of string property parsing so that it treats the whole property value as a single string in case that the property allows only one value.

clean Model a little bit
Un-duplicate comments for properties.
Fix typo

[ci skip]
Fix typo in comment.

[ci skip]
Made modifications according to comments for the PR
Merge branch 'hackathon_Justin_7_17' into hackathon_Justin
Hold onto Output's and Input's using unique_ptrs.
Remove comparisons of int to NULL.

Fixes clang warnings introduced by the use of ReferencePtr.
Convert Model Output labels to lowercase_
Merge pull request #106 from opensim-org/hackathon_Justin

Update Model class to match new coding conventions of ModelComponent. Merging now, but there are still an issue with copying of Inputs and Outputs when  a Component is copied. I am going to file as a separate issue for a separate PR.