Add visualisation of regressors

[smoia]

Detailed Description

Add a plot of each regressor computed by phys2denoise before the export.

Context / Motivation

Visualising the regressors is something important!

Possible Implementation

Using phys2bids.viz as an inspiration, output a plot with each regressor as subplot.
The first subplot should be the input.
Separating RETROICOR from everything else would be nice.