Callback closure for printing the result of evaluation
cb.print.evaluation(period = 1)results would be printed every number of periods
The callback function prints the result of evaluation at every period iterations.
The initial and the last iteration's evaluations are always printed.
Callback function expects the following values to be set in its calling frame:
bst_evaluation (also bst_evaluation_err when available),
iteration,
begin_iteration,
end_iteration.