Plot robust loss functions

[pzivich]

Is your feature request related to a problem? Please describe. To RTD, add a plot of the robust loss functions (or rather their derivatives) over the space.

Describe the solution you'd like Create a simple plot (like stats odds & ends or statsmodels documentation).

Describe alternatives you've considered NA

Additional context This applies to v0.6+, where additional loss functions are added.

[pzivich]

this has been added to the upcoming documentation for v0.6