Add Scatter.empty

[nh13]

so we can have:

Scatter().flatMap { result => if (condition(result)) Scatter(result) else Scatter.empty

Also, see the added tests.

Depends on #360

[nh13]

Added
1.
Scatter.apply[Result <: Task](task) wraps a Scatter around a single Task for use with flatMap. So we can do scatter.flatMap { item => if (condition(item)) Scatter(toResult(item)) else Scatter.empty. Note that we could implement Scatter.filter as

def filter(f: Source => Boolean): Scatter[Source] = {
  this.flatMap { source => if (f(source)) Scatter(source) else Scatter.empty }
}

Scatter.apply(Iterable) creates a Scatter from an iterable. It was convenient for (1), but also implemented in #359

[tfenne]

As mentioned in slack, I think you're really redefining the scatter/gather API as a more general "working with collections of tasks" API. Long term I think it's confusing to introduce some of these concepts while maintaining scatter/gather sematics. If we keep pushing in this direction I think we should probably re-write this in terms of a TaskSeq class (which is what Scatter is becoming).

[codecov-io]

Codecov Report

Merging #362 into master will decrease coverage by 0.07%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master     #362      +/-   ##
==========================================
- Coverage   92.03%   91.95%   -0.08%     
==========================================
  Files          31       31              
  Lines        1155     1156       +1     
  Branches       66       65       -1     
==========================================
  Hits         1063     1063              
- Misses         92       93       +1

Impacted Files	Coverage Δ
...ore/src/main/scala/dagr/core/tasksystem/Task.scala	95.52% <50%> (-1.45%)	⬇️

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