Added Avogadro's number and Alpha particle mass

[patchispatch]

Pull request template for Math::Constants

Added Avogadro's number and Alpha particle mass

What kind of constants are you adding and its origin

Avogadro's number and alpha particle's mass are both physical constants.

Check list

• It's a well known constant and I have added a reference for it
• I have added a test and it works.
• I'm using the usual naming and abbreviation conventions.

[JJ]

You can define avogadro by aliasing it to L, which is already there. Please check also syntax: you need to add a semicolon at the end of every sentence, for instance. Equal signs are also missing.