feat: make `From<ChromosomeT>` optional for indivudals in population generators

[kkafar]

Feature description

Implementation of From<IndividualT::ChromosomeT> should be optional for individual type.
So this trait bound should not exist on IndividualT itself, but rather be defined
by operator implementations.

Motivation

As I'm writing solvers for JSP problem it turns out implementation of mentioned trait does not make sense
for every custom individual type making it very problematic to work with population generators.