This MR refactors and stabilizes the configuration system of the codegen module.
It adopts much code from pystencils-sfg!2 (merged), and once it is through, pystencils-sfg can be updated to inherit its config system from pystencils.
Closes #116 (closed)
codegen.config
Option descriptor for config options with default values, validation, and explicit unsettingConfigBase class and Category descriptor for safely modelling config objects and correct implementation of their deep-copyingbackend
codegen.config from the backend module, making it exclusive to configuring the codegen driver