Add tests for current Common functions

[mattgwwalker]

The functions inside Common are not currently tested. Test cases would be desirable.

[mattgwwalker]

I've added tests for about two-thirds of the Common functions. @r0fls would you like to add tests for erfinv() and lsr()? That would just leave factorial() and choose().

[mattgwwalker]

I've just committed tests and documentation for factorial() and choose(). That just leaves the last two.