DEV: Updated spectroscopy.integrate_ROI based on comments in grp meeting

1) made function more general by removing references to energy
   replaced energy references with x-value references.
   energy --> x_value_array
   e_min --> x_min
   e_max --> x_max
2) added value check on x_value_array to make sure that the array
   increases in value, and if the array decreases in value, then
   the array is flipped to correct orientation. This check is followed
   by confirmation that values in x_value_array are equally spaced
3) updated documentation
modulo the case of zeros in the diff