Add a twinx axis to show scaling on optimization variables

[Ipuch]

Add this axis for scaled variables on plots.

Example:

def celsius_to_fahrenheit(x):
    return x * 1.8 + 32

def fahrenheit_to_celsius(x):
    return (x - 32) / 1.8

secax_y = ax.secondary_yaxis(
    'right', functions=(celsius_to_fahrenheit, fahrenheit_to_celsius))
secax_y.set_ylabel(r'$T\ [^oF]$')

[Ipuch]

following #572

[EveCharbie]

The bounds on the plots are not scaled either. So there is a mismatch between the variables (scaled) and bounds (unscaled).