finalize cached file handling

[sebkopf]

• cached files are not currently transferrable, probably because file_root is included in the file hash, don't include it in the hash or figure out why else the caching doesn't work across different file locations
• cached file names should have the file ID in the filename
• there should be a flag in reread_files that allows reread_erroneous_only = TRUE to make it easy to reread erroneous files only in a collection. this replaces the much less useful cache_files_with_errors parameter which achieves the same but has to be called on the original read rather than when re-reading from cache (the more likely practical scenario)
• update handling of different isoreader file versions when reading from .rds, only do backwards compatibility checks if version differences require it to speed up read time (give a warning along the lines of some files were read with an older version of isoreader and require backwards compatibility checks - to avoid this warning, reread_...

[sebkopf]

full implement in version 1.2.0