DEV: spectroscopy.py - Fixed Caswell's comments

1) Added array reversal for both counts and x_value_array in the event that
   the arrays are input running highest to lowest value in the independant
   variable array.
2) Removed the x_value_array spacing test which also resolves Tom's comment
   about not using exact float arithmetic for testing.
3) Added a value check on the x_min and x_max values so that in the event
   that the user assigns the maximum bounds to x_min and minimum bounds
   to x_max, the code corrects for this error automatically without
   raising an exception.