TestFunctions must be improved to work on higher dimensional problems

[joaofix]

At this state, TestFunctions implementation is limited to 2-dimensional problems.
What should be done:

• Update how functions are calculated to support high dimension parameters (50,100, etc)
• Write tests for these functions to assure the correctness of returns (expected input/return lists length)

[joaofix]

Bukin test function is not working properly. bukin(particle) sometimes return nan (why?)