If old external forcing file references a pli file, the accompanying .tim/.cmp should also be read and stored

[priscavdsluis]

          I will create a new issue for the following point (as discussed with team):

The filename value on a filename=... line should maybe ? be of type Union[PolyFile, TimModel, DiskOnlyFileModel].
As such, we hope to recursively read/store if a .pli file is given, but all other file types are only stored by filename, and copied as-is by the existing functionality in DiskOnlyFileModel. The idea behind this is that we need to also read/store for each .pli file the accompanying .tim or .cmp file. Further design discussion is still needed for this.

Originally posted by @priscavdsluis in #369 (comment)