Add notes/Begin section for Combinations (similar to Permutations)
Update notes; change calc_permutations to use the ter 'permutation(s)' instead of 'combination(s)'
Create calc_factorial method; refactor calc_permutations to use calc_factorial; make progress with calc_combinations (presently infinitely-looping - needs fixing)
Resolve infinite-loop in calc_combinations
Move test_statistics to its own directory