Get Stage of a concrete matrix.

        Attributes:
            matrix_name (str): name of the matrix
            stage_name (str): name of the stage.

        Returns:
            CollectorStage: when stage has been found or None.

def get_stage(self, matrix_name, stage_name):
        """
        Get Stage of a concrete matrix.

        Attributes:
            matrix_name (str): name of the matrix
            stage_name (str): name of the stage.

        Returns:
            CollectorStage: when stage has been found or None.
        """
        found_stage = None
        if matrix_name in self.data:
            result = Select(self.data[matrix_name]).where(
                lambda entry: entry.stage == stage_name).build()
            found_stage = result[0] if len(result) > 0 else None
        return found_stage