get_driver is actually a driver factory

[rleander73]

The get_driver method in driver.py (class driver) is a factory pattern returning an instance of a specific driver type based on a string in the config toml.
The selection of the returned instance type currently follows an if-else structurem but maybe better use of pydantic yields a cleaner, less explicit alternative. See imod_python and xugrid for instance.

• string to type conversion automatically (?)
• introspection(?)
• get_driver outside the driver base class in a separate routine