refactor!: Make HClust.cluster arguments clearer

Reading `.cluster(elements, 5)` (5 clusters) is clearer than
`.cluster(elements, 2.134)`, so make cutoff a named argument:
`.cluster(element, cutoff: 2.134)`.

BREAKING CHANGE: Method signatures are changed.

spec/cluster_spec.cr

@@ -30,7 +30,7 @@ describe HClust do
   describe ".cluster" do
     it "returns grouped values" do
       positions = fake_positions
-      HClust.cluster(positions, 4) { |u, v| euclidean(u, v) }.should eq [
+      HClust.cluster(positions, cutoff: 4) { |u, v| euclidean(u, v) }.should eq [
         [0, 3, 6, 7, 9].map { |i| positions[i] },
         [1, 5, 8].map { |i| positions[i] },
         [2, 4].map { |i| positions[i] },
@@ -41,7 +41,7 @@ describe HClust do
   describe ".cluster" do
     it "returns N grouped values" do
       positions = fake_positions
-      HClust.cluster(positions, into: 2) { |u, v| euclidean(u, v) }.should eq [
+      HClust.cluster(positions, 2) { |u, v| euclidean(u, v) }.should eq [
         [0, 1, 3, 5, 6, 7, 8, 9].map { |i| positions[i] },
         [2, 4].map { |i| positions[i] },
       ]

src/hclust/cluster.cr

@@ -49,6 +49,7 @@ end
 # less than or equal to *cutoff*.
 def HClust.cluster(
   elements : Indexable(T),
+  *,
   cutoff : Number,
   rule : Rule = :single,
   & : T, T -> Float64
@@ -68,8 +69,7 @@ end
 # rule *rule* based on the distances computed by the given block.
 def HClust.cluster(
   elements : Indexable(T),
-  *,
-  into count : Int,
+  count : Int,
   rule : Rule = :single,
   & : T, T -> Float64
 ) : Array(Array(T)) forall T