0.4.0

• Support for derived dimensions in exp added
• Added pow() and sqrt() operations on quantities
• units removed from a std::experimental namespace
• Downcasting facility refactored so the user does not have to write the boilerplate code anymore
• From now on base dimensions should inherit from base_dimension class template
• Added unit symbols definitions to base_dimension and derived units
• Added support for operator<< on quantity
• fmt support added
• Derived unit factory helpers refactored
• Refactored the way prefixed units are defined