- IOobjectOption class encapsulates read/write, storage flags for
  lightweight handling, independent of objectRegistry etc.

ENH: add IOobject isReadRequired() and isReadOptional() queries

- encapsulates test of MUST_READ, MUST_READ_IF_MODIFIED,
  READ_IF_PRESENT for convenience / less clutter.

Example,

    if (isReadRequired() || (isReadOptional() && headerOk()))
    {
        ...
    }

Instead of

    if
    (
        (
            readOpt() == IOobject::MUST_READ
         || readOpt() == IOobject::MUST_READ_IF_MODIFIED
        )
     || (readOpt() == IOobject::READ_IF_PRESENT && headerOk())
    )
    {
        ...
    }