feat: Support multi-level paths for custom handlers

[wx-ys]

This PR updates get_named_handler_class to correctly parse handler names with multi-level module paths, such as package.subpackage.ClassName.

This makes the handler loading mechanism more robust and allows for greater flexibility in organizing custom handler code.

For example, if the input is 'galyst.input_handler.ArepoHDFSubfindInputHandler', the module_name will now correctly be 'galyst.input_handler' and the class_name will be 'ArepoHDFSubfindInputHandler'.

Closes #272

[apontzen]

Sorry for the long delay in reviewing this. It looks fine, but is there a way to test it?

[wx-ys]

Hi, I have added a simple test for this.