`impl Add<&Quantity> for Quantity`

[DJDuque]

Is there any particular reason why this is not implemented? And if it is possible, would a PR with this be approved/is of interest?
I currently have a use case for this, so I was wondering if there is any reason it can not be implemented before I dive deep and try to implement it myself for a PR.

[iliekturtles]

Closing as a duplicate of #38. In short, I'm very interested to see this done and welcome contributions, the long of it is that I just haven't had enough time to review and help push #307 to completion.