Load default_model_config_lazily

[cpaniaguam]

This pull request introduces changes to lazily load default model configurations and registered models. The main changes involve replacing direct dictionary access with function calls to improve performance and maintainability.

Key changes include:

Configuration Management:

• Introduced get_default_model_config and default_model_config functions to lazily load and cache default model configurations in src/hssm/defaults.py. [1] [2]

Code Refactoring:

• Updated various functions to use the default_model_config function instead of directly accessing the default_model_config dictionary in src/hssm/config.py, src/hssm/plotting/model_cartoon.py, and tests/param/test_params.py. [1] [2] [3] [4] [5] [6] [7] [8] [9]

Model Registration:

• Updated functions to use the registered_models function instead of directly accessing the registered_models dictionary in src/hssm/register.py and tests/test_register.py. [1] [2] [3] [4]

[AlexanderFengler]

Ok I guess this PR and #655 are linked.