* rename link test to odr test and check odr test completeness
* added convenience header for algorithm
* added minimal docs for axis.hpp, accumulators.hpp, algorithm.hpp
* temporarily remove display.hpp (will return in another patch)
* 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