Add support for "support data"

[smoia]

Closes none but addresses physiopy/prep4phys#19
Depends on #42

This PR add the possibility to plot a secondary (support) physiological wave in the interactive editor.
The secondary wave is not editable, but visualising multiple physiological signals together allows a manual peak edit to be informed by other fluctuations.
A use case is, for instance, peak addition/rejection in a CO2/O2 trace in data without a direct CO2/O2 manipulation.
Plotting a respiratory belt allows to better classify peculiar CO2/O2 fluctuations (e.g. a period of fluctuations within a smaller range than normal) as instrumental artefacts or real changes in respiration.

Proposed Changes

• Add a suppdata (support data) property and parameter in the Physio object, as well as a parameter in the PhysioEditor object.
• Allow plotting of secondary row only in case suppdata contains data.

Change Type

• bugfix (+0.0.1)
• minor (+0.1.0)
• major (+1.0.0)
• refactoring (no version update)
• test (no version update)
• infrastructure (no version update)
• documentation (no version update)
• other

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