Project Euler 62 Solution (#3029)

* Add solution for Project Euler 62

* Add doctests and annotate function params and return values for get_digits()

* Add extra newline between functions to fix flake8 errors

* Add extra newlines between function names

* Add missing return type for solution()

* Remove parenthesis from if statement

* Remove parentheses from while loop

* Add to explanation and fix second Travis build

* Compress get_digits(), add tests for solution(), add fstring and positional arg for solution()

* Remove input param when calling solution()

* Remove test case for the answer