Add solution for Project Euler problem 38. (#3115)

* Added solution for Project Euler problem 38. Fixes: #2695

* Update docstring and 0-padding in directory name. Reference: #3256

* Renamed is_9_palindromic to is_9_pandigital.

* Changed just-in-case return value for solution() to None.

* Moved exmplanation to module-level docstring and deleted unnecessary import