Memory leak fixes in the wrapper

[vineetbansal]

Thanks to @XiDianZuoYun for bringing this to our attention through issue 198.

Getting the the .solution property of a PyOSQPSolver calls .get_solution(), which in turn does this:

    PyOSQPSolution* solution = new PyOSQPSolution(*this->_solver->solution, this->m, this->n);
    return *solution;

This created a new PyOSQPSolution on the heap every time it was called and returned a reference to it. Since ownership was never transferred and the pointer was never stored, this leaked memory on every call.

The solution is now cached using a std::unique_ptr member variable. The object is created once on first access and automatically cleaned up in PyOSQPSolvers destructor (which calls osqp_cleanup).

Another unrelated memory leak was related to:

this->_solver = new OSQPSolver();

The actual solver is allocated elsewhere (by osqp_setup), so this allocation was unnecessary and would have leaked.

I've tested out the fixes using valgrind on a script that repeatedly access .solution. Something like this:

    for i in range(iterations):
        _ = prob._solver.solution.x
        _ = prob._solver.solution.y

While this was leaking memory in the original implementation, the leak is fixed with this bug.