openfoam: improve input mesh checks and internal handling

- check for illegal cell ids immediately when owner/neighbour are read.
  Handle ancient format, where neighbour had same length as owner
  but padded with '-1' as per OpenFOAM.
  Truncate neighbour list immediately so that the resulting lists
  correspond exactly to nFaces and nInternalFaces.

  With this done on input, makes various later sanity checks redundant,
  simplifies looping, can determine cell-face allocations and
  reconstruct cells anytime later.

- make cell-face construction demand-driven.