- 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