validate -> validate_internal

[ghost]

It seems a little ridiculous to have
validate(X, y, method, options)
and
validate_repeat(X, y, method, options, n_trials = 100)

We should consider renaming validate_repeat -> validate and validate -> validate_internal (not exported to users) throughout all of the code. Then users can just specify n_trials in validate(). Maybe this defaults to 1 then instead of 100?