allow re-read of .rda collections about isoreader

[sebkopf]

still missing

• the re-read that does this directly for an .rda file
• allow passing in a new root directory for files that were relocated

not implemented (deemed not important): re-read with the original file read parameters of individual files because file read parameters are going to be on their way out at some point anyways in favor of trying to read everything and working with cached files for speed

[sebkopf]

the last point (new root directory) can now be achieved most easily with iso_set_file_root