Add a custom gate application for Toffoli in default.qubit

[antalszava]

DefaultQubit uses a custom implementation for certain gates. These gates are contained in the _apply_ops dictionary of the device. As an example, to perform a PauliX bit flip, default.qubit calls the np.roll function on the state vector.

We would like to add a custom gate definition for the Toffoli gate. This will be similar to how the custom application of PauliX and CNOT is being done.

Sketch for the steps of the addition:

1. In the pennylane/devices/default_qubit.py file, extending the _apply_ops dictionary to contain the "Toffoli": self._apply_toffoli entry,
2. Defining the _apply_toffoli method for DefaultQubit in the same file (the example of CNOT can help, good point of reference is the _apply_cnot function),
3. Adding a test case for applying the Toffoli gate in devices/test_default_qubit.py.