RFC: one(x::SIQuantity{T}) returns T

In some discussion in #18 that happened after #30 was merged, it was suggested that `one(x::SIQuantity)` should return a basic number type (i.e. the result of `one(x.val)`) rather than an `SIQuantity`. This PR changes that, and adds a few tests.

I'm not 100% certain this behavior is better than the previous, but at least this way it's easy to compare them, and this PR can serve as the basis for a concious decision.