Pair class

This merge request completes the review of the Pair class,

• Doc strings
• License text
• Unit tests

I have noticed that both the Pair class and the Channel class define the constants hbarc and alpha in their header files,

    static constexpr double hbarc = 197326.9788;       /**< Reduced Planck's constant times speed of light in units of keV.fm **/
    static constexpr double alpha = 7.2973525664e-3;   /**< Fine structure constant ~1/137. **/

It seems a little silly to be defining the same natural constants in several places of the code. I'm not entirely sure why this is done in this manner, but I believe it may allow the constants to be assigned values already at compilation time:

https://en.cppreference.com/w/cpp/language/constexpr

Not sure if/why this is needed ...

Your thoughts?