C++ improvements

[bluescarni]

This PR adds a couple of capabilities on the C++ side:

• generic assignment of a UDP/UDA/... to a problem/algorithm/..., so that now it is possible to write:

problem p;
p = rosenbrock{};

• a new function is_valid() for user-defined classes, with which it is possible to query if the object was moved from:

problem p;
assert(p.is_valid());
problem p2 = std::move(p);
assert(!p.is_valid());

We already stated in the documentation that moved-from objects could only be destroyed and assigned to (revived). Now it is also possible to call is_valid() to query the current state of the object.

None of these features are meaningful in Python, so pygmo was not touched by these additions.

I also moved null_problem and null_algorithm to their own headers (they previously resided in problem.hpp and algorithm.hpp), and made some other smaller internal changes.

[darioizzo]

Looks good to me... happy about the null prob algo move !!