Add leaf interface for consistency

src/main/kotlin/com/tpcly/behaviourtree/node/Action.kt

@@ -13,7 +13,7 @@ import com.tpcly.behaviourtree.TreeNodeResult
 class Action<S>(
     override val name: String,
     val func: (state: S?) -> Status
-) : TreeNode<S> {
+) : Leaf<S> {
     override fun execute(state: S?): TreeNodeResult<S> {
         return TreeNodeResult(this, func(state))
     }

src/main/kotlin/com/tpcly/behaviourtree/node/Condition.kt

@@ -11,7 +11,7 @@ import com.tpcly.behaviourtree.TreeNodeResult
 class Condition<S>(
     override val name: String,
     val predicate: (state: S?) -> Boolean
-) : TreeNode<S> {
+) : Leaf<S> {
     override fun execute(state: S?): TreeNodeResult<S> {
         val status = when (predicate(state)) {
             true -> Status.SUCCESS

src/main/kotlin/com/tpcly/behaviourtree/node/Leaf.kt

@@ -0,0 +1,3 @@
+package com.tpcly.behaviourtree.node
+
+sealed interface Leaf<S> : TreeNode<S>

src/main/kotlin/com/tpcly/behaviourtree/node/Perform.kt

@@ -10,7 +10,7 @@ import com.tpcly.behaviourtree.TreeNodeResult
 class Perform<S>(
     override val name: String,
     val func: (state: S?) -> Unit
-) : TreeNode<S> {
+) : Leaf<S> {
     override fun execute(state: S?): TreeNodeResult<S> {
         func(state)
         return TreeNodeResult.success(this)