Conversions work from arbitrary Numerics.
Merge pull request #46 from transcriptic/int-conversions

I like this very much.  However, I'd like to a bit further and allow the apply methods of the Units to accept any Numeric as well.  This will make the entire API flexible enough to accept any Numeric.

One goal of the project is to replace the underlying Double with a generic Numeric, giving user code the ability to use whatever type they want.  Of particular interest are the types in the Spire project.  That turns out to be pretty involved refactor, however, supporting Numeric in the API is good first step there.  It will allow user code to immediately begin initializing with other types.

I am going to merge this PR, but before you move on to other types I will adjust the pattern as described.

Thanks.