C++ writer for component implementation templates

[tiffany1618]

Closes #218

• Add ComponentImplWriter for writing C++ component implementation templates
• Add template parameter to CppWriterState to indicate whether to generate component base or implementation classes
• Add support for generating C++ files with arbitrary file extensions
• Reorganize unit tests for C++ component writer and add unit tests for implementation templates

Changes from Python autocoder:

• Add handler implementations for internal interfaces
• Change file extension of template files from .{hpp,cpp}-template to .template.{hpp,cpp}
• Fix formatting of Doxygen comments