This clarifies how to use histogram::fill efficiently with multi-dimensional histograms, and discusses a workaround for the design choice that Axis::index cannot be overloaded.
And since I haven't build the documentation locally for a long time, and a new computer where I needed to set things up again from scratch, I also added a README.md on how to do that (which is not trivial for boostbook).
- added axis::traits::continuous to match axis::traits::is_continuous
- axes now return mutable metadata even when constant
- histograms with discrete growing axis can be added
- use multi_index in histogram interface
* InputIterator concept requires that the decrement of a postfix-incremented iterator is valid, now it is
* added detached_accessor to that end
* update benchmark plot, better docs
