Add full rotation to units

[rprospero]

As part of looking into #173, it was noticed that we did not have an angular unit that represented a full rotation. To that end, I've added a rotation unit that is equal to 360° or 2π radians.

One major advantage, captured in a couple of new unit tests, is we can now explicitly set out interpretation of inverse time units.

units.hertz # inverse time with no angular component
units.rotations * units.hertz # rotational frequency  cos(2π f t)
units.radians * units.hertz # angular frequency cos(ω t)

As part of this process, the Unit class was extended to support more variations of scalar multiplication and division. For example, while we previously supported 60/units.minutes, an exception was thrown on 60*units.seconds or units.minutes/60

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[DrPaulSharp]

All looks good.