Included `ostream` as the 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 this header do not have to explicitly include `<ostream>` or `<iostream>` 
before this header. In other words, this makes this header self-sufficient.