harmonize plot phase colorbar

[ahms5]

When I was reviewing #242, I've noticed that the phase is sometimes from -pi to pi, or from 0 to 2pi.

I was thinking that it would be nice to harmoize it. to force the range in cbar from -pi to pi and also add labels relative to pi, similar to pyfar/pyfar#540

[hoyer-a]

The baseline plots showing -1.5 to 1.5 are testing the limits parameter, these are the limits passed to the plot function.
By default it is the data range, in this case 0 to 2pi.

[hoyer-a]

but +1 for the labels relative to pi

[ahms5]

In pyfar (develop) we currently wrap it form -pi to pi, I would suggest sth limilar here for the phase colorbar:

import pyfar as pf
impulse = pf.signals.impulse(100, 10)
pf.plot.phase(impulse)

[mberz]

In the context of phase on the sphere (and spherical harmonics) the phase is shown in equivalence to the azimuth angle.

Definition of the complex valued spherical harmonics:

$Y_n^m(\theta,\phi) = N_n^m P_n^m(\theta) e^{im\phi}$

Since the azimuth angle is defined on the interval $[0, 2\pi]$ is intuitive if phase is shown like that as well.

Maybe it makes sense here to allow specifying options to support different phase wrapping/unwrapping methods like in pyfar.

[mberz]

We decided to

1. keep the interval $[0, 2\pi]$ but use the phase tickers and formatters implemented in pyfar for the colorbar tick labels.
2. not implement phase unwrapping at this point.