Added a solution for Project Euler Problem 203 "Squarefree Binomial Coefficients" (#3513)

* Added a solution for Project Euler Problem 203 (https://projecteuler.net/problem=203)

* Simplified loop that calculates the coefficients of the Pascal's Triangle. Changes based on review suggestion.

* Moved get_squared_primes_to_use function outside the get_squarefree function and fixed a failing doctest with the former.