check_string: Check that a specific list element is a string
In a-maldet/readall: Read FWF, DSV, EXCEL And SAS Files

Check that a specific list element is a string

check_string(
  obj,
  attr,
  err_h,
  allowed_values = NULL,
  allow_na = FALSE,
  allow_null = FALSE
)

`obj`	The list object, that should be checked
`attr`	A string holding the name of the list element, that should be checked.
`err_h`	An error handler.
`allowed_values`	An optional set of allowed strings.
`allow_na`	Are `NA` values allowed?
`allow_null`	A logical, defining whether the argument is allowed to be `NULL`.