- 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
* introduce offset for faster linearization of non-growing axes
* added traits::is_inclusive and constexpr bool inclusive() methods for builtin axes
* bug fixes for fill method when weight array is used with non-inclusive axis and when growing axes are used
* bug fix of axis::options::test(...)
* coverage tested with gcc-8, updated CONTRIBUTING.md with coverage info