New methods added to `electrical_signal` class

[armando-palacio]

✨ Features:

• __radd__ for right sum operand. Same that __add__.
• __sub__ for left substract operand.
• __rsub__ for right substract operand.
• __rmul__ for right product operand. Same that __mul__.
• __lt__ for the less-than comparison.

🙌 Bump Version to 0.4.15