Implement Functions in C#

[ikeough]

The c# language generator will generate function stubs in Functions.g.cs pointing to implementations, but the implementations will need to be authored by hand. I did a review of the amount of effort required to generate these functions automatically and it amounts to writing a full code transpiler from the function language used in the IFC EXPRESS schema to the target language. Although this would be great, and is supported by the current grammar, it seems like we can make a one time implementation of these functions manually, then commit them.