input: Write a function to return an array with the running total of numbers in a given list using the numpy library. If the input list is empty, return an empty array. The function should be called `runningTotal` and take one parameter `arr`, which is a list of numbers. 
  assert np.array_equal(runningTotal([1, 2, 3, 4]), [1, 3, 6, 10])
  assert np.array_equal(runningTotal([5, 5, 5, 5]), [5, 10, 15, 20])
  assert np.array_equal(runningTotal([1]), [1])
  assert np.array_equal(runningTotal([]), [])
  assert np.array_equal(runningTotal([-1, 2, -3, 4]), [-1, 1, -2, 2])
output: import numpy as np
  
  def runningTotal(arr):
      if not arr:
          return np.array([])
      return np.cumsum(arr)