ENH: support ASCII List output on a single-line

- Introduce writeList(Ostream&, label) method in various List classes to
  provide more flexibility and avoid hard-coded limits when deciding if a
  list is too long and should be broken up into multiple lines (ASCII only).

- The old hard-code limit (10) is retained in the operator<< versions

- This functionality is wrapped in the FlatOutput output adapter class
  and directly accessible via the 'flatOutput()' function.

Eg,
    #include "ListOps.H"
    Info<< "methods: " << flatOutput(myLongList) << endl;

    // OR

    Info<< "methods: ";
    myLongList.writeList(os) << endl;