merge CSV files

Merge a set of CSV files containing time series data into a single CSV file containing all the data. ## Deliverables Merge the CSV files attached by date, keeping only the "Close" column. When data for a certain date is not present in a file, enter "-1". In the merged file, label the column corresponding to data from a file such as "[login to view URL]" by the three characters beginning with "Z", for example Z08. Write the code so that it easy to merge other columns of the file by changing "Close" to something else.