Treat scales as unity if they are offset from unity at machine precision
Only apply the is_effectively_unity check when assigning the ._scale internal attribute, making it unncessary to use whenever comparing .scale to 1.  This still places onus developers of the units package to remember this check whenever updating ._scale, but that's less trouble than the current case.
Merge pull request #1407 from mhvk/units-scale-near-unity

Treat scales as unity if they are offset from unity at machine precision - solves #748