docstr: ...Test HawkesKernelSumExp string representation
          
code: def test_HawkesKernelSumExp_str(self):
      """...Test HawkesKernelSumExp string representation"""
      self.assertEqual(
          str(self.hawkes_kernel_sumexp),
          "0.3 * 1 * exp(- 1 * t) + 4 * 2 * exp(- 2 * t) + "
          "2 * 0.2 * exp(- 0.2 * t)",
      )
  
      self.assertEqual(
          str(self.hawkes_kernel_sumexp),
          "0.3 * 1 * exp(- 1 * t) + 4 * 2 * exp(- 2 * t) + "
          "2 * 0.2 * exp(- 0.2 * t)",
      )
  
      self.decays[1] = 0
      self.intensities[2] = 0
      hawkes_kernel_sumexp = HawkesKernelSumExp(
          self.intensities, self.decays
      )
      self.assertEqual(
          str(hawkes_kernel_sumexp), "0.3 * 1 * exp(- 1 * t) + 4 + 0"
      )
  
file_path: tick/tick/hawkes/simulation/tests/hawkes_kernel_sum_exp_test.py
imports: import unittest
  import numpy
  from tick.hawkes import HawkesKernelSumExp
label_desc: Use image node_id 5 for calling the Test obj's underlying member method code with example usage: obj.test_HawkesKernelSumExp_str() without return types