FMIL: Begin to implement parsing of `SourceFiles` for #3267. Add representation in model description and expose to the user, and skeleton for parsing in the ME case.
FMIL: Factor out code to push back a string onto a vector of strings so that we can reuse it for parsing `SourceFiles`, for #3267.
FMIL: Add parsing of `SourceFiles` for the ME case, for #3267.