Prep code for Jan 2019 Hackathon

[Steven Kopsa]

Below are the items that we want to have complete before the Hackathon, in descending priority order:

Tasks for @stevk :

• Code style guide
• Contributing guide
• Create a ModelGroup class template
• Update existing code to be compliant with the code style doc
• #429 Add a Negative test flag to the manifest
• Test building the project in Visual Studio Code
• #441 Make a model to handle asymmetric, non-zero tangents for cubic spline

Tasks for @kcoley :

• #468 Change how the folder index is tracked so it doesn't change when model groups are inserted
• Test building the project in a Mac environment