Feature/to unique ptr

Intermediate replacement for std::unique_ptr used in return types of some factory methods where currently raw pointers are returned. Main features are: three conversion methods to raw-pointer, unique_ptr and shared_ptr.

Example of usage:

int* f_old() { return new int(1); } // old interface
to_unique_ptr<int> f_new() { return new int(2); } // new interface

// user code:
int* x1 = f_old();
int* x2 = f_new(); // works, because of implicit conversion to int*

std::unique_ptr<int> y1 { f_old() };
std::unique_ptr<int> y2 { f_new() };
std::unique_ptr<int> y3 = f_new();

The conversion to raw pointers is marked deprecated, so that the class to_unique_ptr can be replaced by unique_ptr in later releases.