simplify class structure modelling & update class composer

src/pystencilssfg/composer/basic_composer.py

src/pystencilssfg/composer/class_composer.py

src/pystencilssfg/composer/mixin.py

src/pystencilssfg/context.py

src/pystencilssfg/ir/analysis.py

src/pystencilssfg/ir/call_tree.py

src/pystencilssfg/ir/source_components.py