DEV: Added unittests for mods to spectroscopy.py

1) Added 2 tests to test_integrate_ROI_compute()
   a) Evaluate case where x_value_array, and compute are entered
      with monotonically decreasing x_array values
   b) Evalute case where all input parameters are entered "backwards"
2) Commented out unit test where all x_min and x_max values are flipped
   since a correction for this case has been added to the code resulting
   in this case NOT resulting in an exception error.