mini refactoring and Rd53a emulator revival

This MR is intended for mini refactoring for better practice.

• Introduce constexpr (i.e. using ROM, meaning that the value of these variables are determined at the compiling stage) where it seems appropriate.
• Unit constants (e.g. 1.e-15 == Unit::Femto or 1.6e-19 == Physics::ElectronCharge ) were moved to dedicated namespaces as constexpr.
• Hiding private members of likely-final derived classes into ::Impl class (this Impl class is only forward-declared in the header, and the class definition is in the .cpp file) where it seems appropriate. This is the so-called pImpl idiom (pointer-to-implementation). An example of this is Rd53aCoreColLoop.h
• Moving implementations to .cpp files
• Change NULL to nullptr where I noticed.
• Using initializer (and initializer list) where possible.
• Writing override and final where it seems possible (not comprehensive)