* 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
- Tests and benchmarks now only include the headers they really need.
- Warnings turned off for tests that compile with external boost libs that are not warning free
* provide unsafe access to buffer
* factored mp_int out into separate header and renamed to large_int
* refined adding behavior
* more warnings and comeback of coverage variant
* better coverage reporting
* 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
