- previously had 'mandatory' (bool) for advanced control of reading
  dictionary entries but its meaning was unclear in the calling code
  without extra code comments.

  Now use IOobjectOption::readOption instead, which allows further
  options (ie, NO_READ) and is more transparent as to its purpose in
  the code than a true/false bool flag was.

  This is a minor breaking change (infrequent, advanced usage only)

- minor code cleanup in dictionary lookup methods