Included <ostream> as ScientificQuantities.hpp depends on it. 

The header `ScientificQuantities.hpp` now includes `<ostream>` as the code 
depends on `std::ostream` to be available. Including this header ensures that 
users of `ScientificQuantities.hpp` do not have to explicitly include `<ostream>` 
or `<iostream>` before it. In other words, this makes `ScientificQuantities.hpp` 
self-sufficient.