Create visualization of all test functions in argmin_testfunctions

[stefan-k]

It would be convenient to have a plot of each test function in the argmin_testfunctions API docs. I'm not sure what the best approach would be, but probably a binary which uses plotters to produce the plots is reasonable. I'm open to other suggestions. For each test function it should be possible to define the domain and the orientation of the plot. Finally the plots need to be added to the docs.