DEV: Finished updating integrate_ROI per Tom's github comments

Finished updating ValueError messages using .format()
Removed auto-correction of max/min boundaries so that ValueError
exception is raised instead.
DEV: Added test case for reversed x_value_array and counts input

The test verifies that if both the 'x_value_array' and 'counts' data sets
are input in reversed order (so that the values are decreasing monotonically)
then both arrays are flipped so that the independent variable values meet
the "monotonically increasing" required condition.
In the event that this reversal occurs, a print statement is generated in
order to notify the user that this reversal has occurred.