* 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
- new thread-safe accumulators for arithmetic types
- storages have a new flag `has_threading_support` to indicate support for parallel cell access
- histogram automatically synchronises growing axes and storage if necessary
* concept docs rewritten, adapting to tabular style as in Beast and cppreference.com
* update the readme with correct links and text
* removed necessity to add `storage_tag` member type to storage classes
