Uniform usage of ratio as boost::ratio

[traversaro]

While debugging a strange ABI issue in a downstream Boost Chrono package (RoboStack/ros-noetic#448), I found an inconsistencies between std::ratio and boost::ratio. I was not able to reproduce the issue, however I noticed that in Boost Chrono there are some inconsistency in the use of ratio, that sometimes is used as boost::ratio, and sometimes is used just ratio (this is probably a leftover from the time where std::ratio did not exist).

Regardless of the root issue I was experiencing, I think (but I may be wrong) that it may be beneficial to uniform the usage in the library to always use boost::ratio, for consistency and avoid confusion with std::ratio .