openalexR 3.1.0

• New oa_options() constructor for building the options argument of
  oa_fetch() and oa_query(). It provides argument autocompletion, defaults,
  and validation (e.g. catches misspelled options and out-of-range
  sample/per_page),
  while a plain list() is still accepted for backward compatibility (#182).
• Paging parameters per_page, paging, and pages are now set through
  options = oa_options(...). Passing them as top-level arguments to
  oa_fetch() is deprecated but still works (with a warning).
• oa_fetch() now errors when more than one filter has over 50 values, since
  only one such filter can be chunked per call.