* reducible category axis
* new axis::trait is_ordered and ordered()
* reduce accepts positional commands now
* renamed (old symbols still there, but deprecated)
- static_is_inclusive -> is_inclusive
- static_options -> get_options
- reduce_option -> reduce_command
* reduce commands are now structs derived from reduce_command base instead of functions
* simplified TMP code in axis::traits
* updated and improved docs for reduce
* 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
