* 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
* rename get_size to axes_rank and move axes buffer to axes.hpp
* added fill experiments
* faster and simpler fill_impl
* faster specializations for linearize_value
* renaming naked to remove_cvref_t
* more static_asserts to check implicit conditions
* use new BOOST_TEST_TRAIT_SAME
* reorganize and add operator tests, fixes for array and map adaptors
* improved map adaptor
* storage does not have to support scaling operator anymore
* doc improvements
