future_epw() should return information about how the weather data is split

[hongyuanjia]

Currently, future_epw() directly returns the created Epw objects for future climate. I always find I have to do manual steps to process each generated EPW file name using regex to get an idea of the scenario of each output. It would be useful to return a data.frame containing the information about how the data is split and aggregated based on the by argument.