Improve the optimization unit test

[accosmin]

Split the check in multiple smaller parts:

• check first that all optimization methods decrease the loss value (e.g. f - 0.5 * |f|)
• check convergence
• check solutions (if known, aka !solutions.empty())

[accosmin]

The problem is that some test functions are not bounded in some directions (e.g. Beale). So in this case, the function_t interface should be extended to contain a function that checks if a point is within bounds.

Replace function_t to a more object oriented design.