WIP patching the remaining code with the new config system

src/pystencilssfg/__init__.py

src/pystencilssfg/config.py

src/pystencilssfg/context.py

src/pystencilssfg/emission/__init__.py

src/pystencilssfg/emission/emitter.py

src/pystencilssfg/emission/header_impl_pair.py

src/pystencilssfg/emission/header_only.py

src/pystencilssfg/generator.py

tests/generator/data/project_config.py

tests/generator/test_config.py