Exposing domain() of interpolation in pchip Class

[rodv92]

It seems that domain() is not exposed in pchip (and maybe some other interpolators).

Adding :

std::pair<Real, Real> domain() const
{
    return impl_->domain();
}

to the class would allow to perform soft checks of the domain and avoid throwing an exception when abscissa x is out of bounds

I tested by adding the above snippet to the class with no issues, while using an underlying
boost::math::interpolators::pchip<boost::circular_buffer> implementation.

unless that is by design or the consequence of some other issue, that would be really useful.

[mborland]

Yes, we can add this in.