Add support for in-model libraries

[finger563]

Allow a new type of library which can contain 0..* header files and 0..* source files. The library will be generated as a ROS package that can be used just like any other library, except the code will live in the model and will therefore be able to be compiled without needing an internet connection and simpler to modify as needed.

[finger563]

The main drive for this is partially handled by #219 which no longer needs an internet connection and is easy to modify - the downside being that the package / code is not easy to distribute with the model and not easy to version.

[finger563]

After we finalize the implementation of this feature, we should look into developing an importer plugin which can take an existing package as input (either location (web/local) or zip file) and generate the in-model library from it.