read_iso_file: Read individual iso file
In isoreader: Read Stable Isotope Data Files

Low level read function for an individual iso file. Usually not called directly but available for methods development.

read_iso_file(
  ds,
  root,
  path,
  file_n,
  files_n,
  read_from_cache,
  read_from_old_cache,
  reread_outdated_cache,
  write_to_cache,
  cachepath,
  old_cachepath,
  post_read_check,
  ext,
  reader_fun,
  reader_options,
  reader_fun_env
)

`ds`	the basic data structure for the type of iso_file
`root`	root directory for the isofiles. Can be relative to the current working directory (e.g. `"data"`) or an absolute path on the file system (e.g. `"/Users/..."` or `"C:/Data/.."`). The default is the current working directory (`"."`). Can be supplied as a vector of same length as the provided paths if the paths have different roots.
`path`	file path
`file_n`	number of processed file for info messages
`files_n`	total number of files for info messages
`read_from_cache`	whether to read from cache
`read_from_old_cache`	whether to read from old cache files (to be deprecated in isoreader 2.0)
`reread_outdated_cache`	whether to reread outdated cache files
`write_to_cache`	whether to write to cache
`cachepath`	path for the cache file
`old_cachepath`	path for the old cache files
`post_read_check`	whether to run data integrity checks after a file read
`ext`	file extension
`reader_fun`	file reader function
`reader_options`	list of parameters to be passed on to the reader
`reader_fun_env`	where to find the reader function