Improve the integration of UVEC and custom UVEC

[brunozc]

Improve the post_init for the UvecLoad in load.py:

` def post_init(self):
"""
Check if the UVEC model is supported in STEM.
"""

    if self.uvec_model is not None:
        if self.uvec_model.__name__ not in (model.value for model in UvecSupportedModels):
            raise ValueError(f"UVEC model {self.uvec_model} is not supported. Please use one of the following models: \
                    {[model.value for model in UvecSupportedModels]}")
        self.uvec_file = "/".join([self.uvec_model.__name__.split(".")[-1], "uvec.py"])
        self.uvec_function_name = "uvec"`