Add relative difference to pyam.compare

[phackstock]

As part of the region-processing done by nomenclature there can be the comparison between model native and aggregated results. This comparison is done using pyam.compare. The output of this function is a pandas.DataFrame which contains two columns with the different results for the two pyam.IamDataFrame objects that are compared. What would be convenient, is to add a third column which contains the relative difference. This way it can be sorted so that differences can be inspected in order.