ENH: reduce overhead and clearer data ownership for OFstreamCollator

- local data to be written is now transferable into the
  OFstreamCollator. This avoids making a full copy when threading is
  active.

- use plain lists for managing proc data
  * storage: List<List<char>> instead of PtrList<List<char>>
  * views:   List<stdFoam::span<char>> instead of PtrList<SubList<char>>

- use gather/write (unthreaded) as backstop if the output is too big
  to fit in the buffer size. Emit warning instead of FatalError