ENH: improved isolation of MPI access in Pstreams

- refactor as an MPI-independent base class.

  Add bufferIPC{send,recv} private methods for construct/destruct.
  Eliminates code duplication from two constructor forms and reduces
  additional constructor definitions in dummy library.

- add PstreamBuffers access methods, refactor common finish sends
  code, tweak member packing

ENH: resize_nocopy for processorLduInterface buffers

- content is immediately overwritten

STYLE: cull unneeded includes in processorFa*

- handled by processorLduInterface