Phasor representation design

The current implementation of Phasor representation has a design flaw, as it is surprising (the second case of the namespace is not the second argument of input) and suboptimal (instead of storing the input amplitudes, the current implementation restores them through computation).

This design was made to guarantee the correct inheritage of FrequencySeries arithmetical methods in the Phasor class, although the Phasor is in fact forbidden to call such methods.

To consider if still maintain the Phasor as a subclass of FrequencySeries (with some consequences on type hinting).