Implement TruncableSteps.__setitem__ for slice, and int

[alexbrillant]

    def __setitem__(self, key: Union[slice, int, str], new_step: BaseStep):
        """
        Set one step with a key, and a value.

        :param key: slice, index, or step name
        :type key: Union[slice, int, str]
        :param new_step: step
        :type new_step: BaseStep
        """
        if isinstance(key, str):
            index = 0
            for step_index, (current_step_name, step) in enumerate(self.steps_as_tuple):
                if current_step_name == key:
                    index = step_index

            new_step.set_name(key)
            self.steps[index] = new_step
            self.steps_as_tuple[index] = (key, new_step)
        else:
            # TODO !! issue 
            raise ValueError('type {0} not supported yet in TruncableSteps.__setitem__, please implement it if you need it'.format(type(key)))