Speed up search knn

[betterenvi]

What this PR does:

• Speed up _search_node

  • Since the 0th element of a heap is the smallest one, we can use heap[0] directly instead of min(heap).
  • In _search_node, when checking whether to search the other side of a plane, the original condition is always true.

• Add code to run test script

[stefankoegl]

Merged, thanks!