- 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
* 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
* Travis uses b2 and codecov now
* replacing boost::multiprecision with custom implementation to avoid the dependency
* improved axis implementation: `update` is now a normal method
* introduced use_default tag type to set defaults
* whitespace fixes and doc update
