Functional testing structure

[ben18785]

In discussions with @DavAug @rcw5890 @MichaelClerx @fcooper8472 we decided to implement functional testing functionality in the main PINTS repo in the following way:

• have a functional_testing folder
• in its __init__.py file create functions for each toy problem which takes as inputs: the sampling method, optional hyperparameters, optional # of iterations, optional # of chains
• within the folder, each sampler has its own .py file which contains test_* methods for each of the toy problems.
• these test_* methods would be discoverable in automated fashion