Compiling against Apache Spark 2.3.2: fix incompatibilities

[JulienPeloton]

What this PR brings?

Recent version of Spark changed the input type (Iterator -> Iterable) for ordering.leastOf. The code now reads (utils/Utils.scala):

private def takeOrdered[T](input: Iterator[T], num: Int)(implicit ord: Ordering[T]): Iterator[T] = {
    val ordering = new GuavaOrdering[T] {
      override def compare(l: T, r: T): Int = ord.compare(l, r)
    }
    ordering.leastOf(input.toIterable.asJava, num).iterator.asScala
  }

How this has been tested?

Unit tests pass.

[JulienPeloton]

Oh, and the Spark version in the build.sbt has been updated: 2.1.0 -> 2.3.2.

[codecov-io]

Codecov Report

Merging #107 into master will not change coverage. The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #107   +/-   ##
=======================================
  Coverage   96.37%   96.37%           
=======================================
  Files          32       32           
  Lines        1240     1240           
  Branches      218      218           
=======================================
  Hits         1195     1195           
  Misses         45       45

Flag	Coverage Δ
#python	94.3% <ø> (ø)	:arrow_up:
#scala	97.24% <100%> (ø)	:arrow_up:

Impacted Files	Coverage Δ
src/main/scala/com/spark3d/utils/Utils.scala	97.36% <100%> (ø)	:arrow_up:

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