`mil_vision.ContourClassifier` ABC has a type error

[cbrxyz]

The mil_vision.ContourClassifier has a type error in the read_from_csv method:

    def read_from_csv(self, training_file: Optional[str] = None):
        """
        Return features and classes from specified training file.
        """
        training_file = _get_param(training_file, self.training_file)
        df = pandas.read_csv(training_file)
        classes = df.values[:, 1]
        features = df.values[:, 2:]
        return features, classes

Currently, the ABC allows a None value to be input for the training_file parameter (this makes sense since it's a non-implemented ABC). However, this causes a few type issues. The pandas.read_csv file will break if None is passed in. Therefore, this needs to be fixed.

One solution could be adding raise NotImplemented if training_file == None. This would be expected from an ABC, and would mitigate the error. If this is implemented, the child classes will need to be examined to make sure that this won't break any behavior.