Make some handicap_functions methods private

[jatkinson1000]

All except handicap_from_score and print_handicap_table should be private.
To do this requires removing the explicit tests for any private functions and instead testing them implicitly through the above two.

This will require tests writing for print_handicap_table which should bump coverage up.

[TomHall2020]

I wonder, does it make logical sense to remove the explicit tests? It seems unit testing is a perfectly valid reason to break the private convention and use those functions directly.

[jatkinson1000]

This is likely to be addressed in the upcoming PR dealing with #2
At least, it gives a significant shake up to everything and is likely to render much of this moot, I think.