Better conforming with standard notations (C++ and SI)
- Changed spelling of length unit from 'meter' to 'metre' as this is the official SI spelling according to the bureau international des Poids and Mesures
- added SI_ prefix to preprocessor flag to disable ratio conversion to SI_DISABLE_IMPLICIT_RATIO_CONVERSION to scope it closer
- Renamed template parameters to fit with c++ naming convention (See this stackoverflow thread)