Add guessing routines

[Antonio-JP]

Given a sequence (c_n), guessing if it is D-finite is a known problem and is in fact implemented in the package ore_algebra. This issue looks for this problem when we try to guess different types of functions:

• Guess an operator A such that there is g(x) with A . g(x) = 0 and our sequence is given by 1/g(x).
• Guess two operators A and B such that there are functions g(x) and h(x) with A g(x) = B h(x) = 0 and our sequence is given by g(x)/h(x).

This addition indicates that the guessing problem could be an interesting part of the package. Adding a module to gather all the guessing routines would be interesting.

• Gather all guessing routines in the module.
• Create a new module guess inside dd_functions.
• Add and test all guessing routines.