input: Write a function which takes an array and a value and returns the index at which this value is present in the array. If the value is not found, return -1.
  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], 2)==0
  assert find_index([2, 5, 9, 15], 15)==3
  assert find_index([2, 5, 9, 15], 7)==-1
output: def find_index(arr, value):
      for i in range(len(arr)):
          if arr[i] == value:
              return i
      return -1