Write a function readfiles() for external files such as plantgrowth_1.txt, plantgrowth_2.txt and plantgrowth_3.txt. It takes two arguments:

• files which is a vector with file names
• labels which can be an optional vector with labels for the research centers

This function should read in all the files mentioned in the first argument files, combine them into a single data frame which also contains a new column origin. The values in origin should depend on what was specified for the argument labels: if some values were specified, then origin should use these as labels for the research centers; if left unspecified, then origin should contain the file names as specified in the argument files. Be sure to include the necessary checks and messages for the user.

There is no need to read in any data files since Dodona will already use plantgrowth_1.txt, plantgrowth_2.txt and plantgrowth_3.txt.