Auto-detect unit prefixes

[Splines]

The lib could automatically detect if an exponent divisible by 3 is used. In this case,

(\num{42.3} \pm \num{1.2}) \cdot 10^{-9} \unit{\V}

it could output the following instead:

(\num{42.3} \pm \num{1.2}) \unit{\nano \V}

If implemented, things to consider:

• If we're very close to a "nice" exponent (divisible by 3), should we round differently? Probably not.
• Have to make sure that "\nano" and the like always works within \unit{...}
• What if the user already specified a unit prefix in the unit string (and still the number uses some exponent)?