`read_files()` only reads one file, not list of files

[danielhuppmann]

As part of the extension to include datetime as time format in addition to yearly data (see #167), the function read_files() was simplified so that it only reads one file.

This function should be extended to take a list of files again, keeping in mind that we now need to check that time_col and extra_cols are compatible, similar to how append() is implemented.