Define `fold` on `Fit` ADT
Define `fold` on `Fit` ADT

Requests were made to add a method to get the fitness value
from a given `Fit` instance. Instead of adding a funtion
specifically to do that, rather introduce a `fold` operation
that will apply the provided function to the data type instead.

Signed-off-by: Gary Pamparà <46654e9f53f0e5c1be5f3914582d375e0aa2156c@gmail.com>

cirg-up/cilib