Allow plot object to be returned from forecast_plots()

[geordie666]

This PR adds a keyword (default of False for backward-compatibility) that, if True, will return the plot object from forecast_plots().

The justification is that, currently, the plot is displayed in certain environments but not all environments. Returning the object allows the plot to be saved (or manipulated in other ways).

[coveralls]

coverage: 30.403% (-0.01%) from 30.413%
when pulling 16baffb on ADM-plot-control
into 5115d9a on main.

[schlafly]

Looks good, thanks!