Move string-printf utility from lmorpho into util. (#228)

Move string-printf utility from lmorpho into util.

* Add `util::strprintf` printf-alike function that returns a `std::string` result.
* Include simple adaptors for `std::string` and standard smart pointer arguments fot `util::strprintf` (i.e. `std::string` arguments can be used with `%s`, smart pointers with `%p`).
* Add unit tests to suit.