Support `boost::rational` as value type for `axis::regular`

[HDembinski]

#include <boost/histogram.hpp>
#include <boost/rational.hpp>

namespace bh = boost::histogram;

int main() {
    using R = bh::axis::regular<boost::rational<int>>;

    auto h = bh::make_histogram(R(10, 0, 1));
}

This currently triggers a static_assert, but should be supported.

[HDembinski]

Related #336

[HDembinski]

Prototype for experimenting

#include <boost/histogram.hpp>
#include <boost/histogram/ostream.hpp>
#include <boost/rational.hpp>
#include <iostream>

int main() {
    namespace bh = boost::histogram;

    struct axis {
        using value_type = boost::rational<int>;
        using index_type = bh::axis::index_type;

        index_type num_;
        value_type offset_;
        value_type scale_;

        axis(index_type n, value_type a, value_type b) :
            num_{n}, offset_{a}, scale_{rat{n} / (b - a)} {}

        index_type index(const value_type& x) const {
            const auto i = boost::rational_cast<index_type>(scale_ * (x - offset_));
            return i >= -1 ? (i < num_ ? i : num_) : -1;
        }

        value_type value(index_type i) const {
            return i / scale_ + offset_;
        }

        index_type size() const { return num_; }

        static constexpr auto options() { return bh::axis::option::underflow | bh::axis::option::overflow; }
    };

    auto h = bh::make_histogram(axis{10, 0, 5});

    h(-1);
    h(1);
    h(2);

    std::cout << h << std::endl;
}