Investigate the use of Generic when constructing an optimization problem

[S-Dafarra]

@diegoferigo mentioned to use generics for better static typing and for having autocompletion when getting a specific structure as input and output.

Related comments:

• Add structure to solve optimization problems #4 (comment)
• Add structure to solve optimization problems #4 (comment)
• Add structure to solve optimization problems #4 (comment)

Reference: https://mypy.readthedocs.io/en/stable/generics.html

[S-Dafarra]

I have done it in 8c10698

For the time being, I just focused on the Problem interface, which is the one used by the user. I did not apply it to the Solver interfaces for ease of use.