Formally handle errors when querying empty streams.

I like this library a lot for its simplicity and ease of understanding,
but I do have a use case where I would like to query an estimator for
a rank without having to know a priori whether it has observed any
samples.  Adding in handling for this a priori do-I-have-samples-and
-avoid-querying-to-prevent-a-panic is dirty at best, so I would like
a way to be lazy about it and check for error value.

I am complately open to your wisdom and reflections on this:

- Representing a response from a query on an empty stream with NaN
  seems fine but also borders on violating the reduction of sentinel
  values to represent state principle and idioms.

  Pros: Concise.  No impact to current API users.
  Cons: Unidiomatic for purists.  NaN may not be good representer of
        illegal state.  NaN check requires importing math module.

- Remodeling Estimator.Get to have a function return signature of a
  double is possible.  (Approach taken in this commit.)

  Pros: Idiomatic for purists.  Minimizes panics in library code.
  Cons: Breaks API compatibility for legacy users.  More verbose.
  Compromise: Users who care about state legality may use
              Estimator.MustGet which has a return signature of a
              single and panics if an error is detected.

Regardless of which approach we take, I think minimizing
unanticipated panics in library code makes everyone happy!  :-)