Bring comments up to date.
Tests for integrate_n_steps.

This required sorting out what Rcpp does with size_t
Little check to be careful accepting size_t from R

Negative numbers will be converted to unsigned integers that
are rather large.  We now route through a function that
throws if the integers aren't positive and then casts to
size_t.
Tests for integrate_adaptive
Tests for integrate_simple, based on integrate_adaptive
Tests for integrate_times