- the very old 'writer' class was fully stateless and always templated
  on an particular output type.

  This is now replaced with a 'coordSetWriter' with similar concepts
  as previously introduced for surface writers (#1206).

  - writers change from being a generic state-less set of routines to
    more properly conforming to the normal notion of a writer.

  - Parallel data is done *outside* of the writers, since they are used
    in a wide variety of contexts and the caller is currently still in
    a better position for deciding how to combine parallel data.

ENH: update sampleSets to sample on per-field basis (#2347)

- sample/write a field in a single step.

- support for 'sampleOnExecute' to obtain values at execution
  intervals without writing.

- support 'sets' input as a dictionary entry (as well as a list),
  which is similar to the changes for sampled-surface and permits use
  of changeDictionary to modify content.

- globalIndex for gather to reduce parallel communication, less code

- qualify the sampleSet results (properties) with the name of the set.
  The sample results were previously without a qualifier, which meant
  that only the last property value was actually saved (previous ones
  overwritten).

  For example,
  ```
    sample1
    {
        scalar
        {
            average(line,T) 349.96521;
            min(line,T)     349.9544281;
            max(line,T)     350;
            average(cells,T) 349.9854619;
            min(cells,T)    349.6589286;
            max(cells,T)    350.4967271;
            average(line,epsilon) 0.04947733869;
            min(line,epsilon) 0.04449639927;
            max(line,epsilon) 0.06452856475;
        }
        label
        {
            size(line,T)    79;
            size(cells,T)   1720;
            size(line,epsilon) 79;
        }
    }
  ```

ENH: update particleTracks application

- use globalIndex to manage original parcel addressing and
  for gathering. Simplify code by introducing a helper class,
  storing intermediate fields in hash tables instead of
  separate lists.

ADDITIONAL NOTES:

- the regionSizeDistribution largely retains separate writers since
  the utility of placing sum/dev/count for all fields into a single file
  is questionable.

- the streamline writing remains a "soft" upgrade, which means that
  scalar and vector fields are still collected a priori and not
  on-the-fly.  This is due to how the streamline infrastructure is
  currently handled (should be upgraded in the future).