[wpiunit] Add Frequency and Measure.inverse()

[WispySparks]

Partially addresses #6683. ~~I'm wondering if it would instead be preferrable to have an inverse() method on Unit that can be overridden by child units. So Velocity and Per would override it instead of having a reciprocal method and Time could override it to give back a frequency. This would then simplify Measure.inverse(). Do you think that's better than the way I've done it now?~~