Add more test functions to `argmin_testfunctions`

[stefan-k]

There are myriads of test functions which could be supported in argmin_testfunctions. Some sources are:

• https://www.sfu.ca/~ssurjano/optimization.html
• https://arxiv.org/pdf/1308.4008.pdf
• https://www.mdpi.com/2306-5729/7/4/46
• http://infinity77.net/global_optimization/test_functions.html#test-functions-index

A few thoughts:

• The implementation should follow the existing implementations.
• Document the actual formula
• For each test function there should also be functions which calculate the derivative and Hessian
• Test functions (and derivative and Hessian) with a variable number of parameters should come in two forms, one where the number of variables is unknown at compile-time (possibly requires allocation on the heap) and a const generics version, which does not allocate.
• Tests
  • Test the optimum
  • Test that derivative at the optimum is 0
  • Test that the derivative (roughly) matches the derivative obtained with finitediff
  • Test that the Hessian (roughly) matches the Hessian obtained with finitediff
• Add benchmarks to benches/testfunctions.rs
• Add function to python interface in ./python/argmin-testfunctions-py and add an example to the readme.

This should be a fairly easy thing to do and is therefore a great starting point for someone who wants to start contributing to argmin.