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
Since array manipulation occurs only because the input arrays were compiled or recorded incorrectly and backwards, and the logging message only applies in this instance, I think that it is important to make sure that notification is provided to the user indicating that the function adjusted and corrected for the input error, but that their source data was backwards. That's why I set the logging rating to "warning" as opposed to "debug." With those considerations taken into account, the description of the warning rating: 
    "An indication that something unexpected happened, or indicative of some problem in the 
    near future (e.g. ‘disk space low’). The software is still working as expected."
seems, to me at least, to be appropriate.
What do you think?