input: Write a function to find the index of a given value in an array. The function should return -1 if the value is not found in the array.
  assert find_index([2, 5, 9, 15], 5)==1
  assert find_index([2, 5, 9, 15], 9)==2
  assert find_index([2, 5, 9, 15], 20)==-1
  assert find_index([], 5)==-1
output: def find_index(arr, value):
      try:
          return arr.index(value)
      except ValueError:
          return -1