Fixed: We need to initilize the _v[_s.length] after setting our partial problem

Else partial problem splitting would not work if solve wasn't called before to initialize _v[_s.length].
Fixed: getPartialProblem always returned the same
Added Testcase for getPartialProblem()

For this there was an parameter added to solve() so it can also act as stepping solver (needed for testing partial problems).