Introduced composer and refactored structure

src/pystencilssfg/__init__.py

src/pystencilssfg/composer.py

src/pystencilssfg/context.py

src/pystencilssfg/generator.py

src/pystencilssfg/source_components.py

src/pystencilssfg/source_components/__init__.py

src/pystencilssfg/source_components/function.py

src/pystencilssfg/source_components/header_include.py

src/pystencilssfg/source_concepts/cpp/std_mdspan.py

src/pystencilssfg/source_concepts/cpp/std_vector.py

src/pystencilssfg/source_concepts/source_objects.py

src/pystencilssfg/tree/__init__.py

src/pystencilssfg/tree/basic_nodes.py

src/pystencilssfg/tree/deferred_nodes.py

src/pystencilssfg/tree/visitors.py

tests/TestSequencing.py

tests/mdspan/kernels.py