degradation_year_on_year should allow center-labeled YoY_values

[cdeline]

Is your feature request related to a problem? Please describe.
At the moment, the YoY_values returned in degradation_year_on_year.calc_info is only right-labeled. This is a problem because to enable #394 (multi-year-on-year calculations) all of the values should probably be center-labeled to be able to line up together correctly. And #455 (time dependent degradation labeling) would benefit from starting with center-labeled YoY_values.

Describe the solution you'd like
Have a kwarg in degradation_year_on_year with label='center'. Current default behavior is label='right'. Consider changing default behavior to label='center'.