* 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
* 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
