Support arbitrary phase-modulated pulses

[HGSilveri]

The ability to define a Pulse with an arbitrary phase modulation has been recurrently requested. This is a working proposition of how this features can be incorporated in the existing codebase with minimal disruption:

• [x] Define Pulse.ArbitraryPhase(amplitude: Waveform, phase: Waveform, ...): Instead of a detuning waveform (ie a frequency modulation), we take a phase waveform, which is then internally converted to a detuning waveform and a phase offset.
• [x] Remove units from Waveform values whenever possible because in a phase waveform the values are not in rad/µs.
• [x] Allow Sequence drawing to show the phase modulation instead of the detuning: this is useful because users will likely want to see the phase waveform they gave to ArbitraryPhase() directly in the sequence (instead of it's detuning + phase offset equivalent)
• [x] Support serialization of parametrized Pulse.ArbitraryPhase() calls in the abstract representation

[HGSilveri]

Do you want to add a property phase_modulation or a method get_phase_modulation to the Pulse class, that would return the phase modulation as a waveform ?

What I'll do instead because I think it's more natural is to add something like that to ChannelSamples. I'll need it for the plotting anyway