Introduce a HamFrequency subclass.

Summary:
Since we want Frequency to be application-agnostic, rather than clutter the
`Frequency` class itself with a lot of ham radio specific stuff, let's
subclass it into a `HamFrequency` class.

Closes T65.

Test Plan:
  scala> val hf = HamFrequency("146.52".MHz)
  hf: com.edgesysdesign.frequency.HamFrequency = 146.520.000

  scala> hf.band
  res3: Option[String] = Some(2m)

Reviewers: jcarter

Reviewed By: jcarter

Maniphest Tasks: T44, T47, T65

Differential Revision: https://phabricator.edgesysdesign.com/D74