tidying compressible wfs - using rasModel to provide fields instead of db lookup, and cache yPlusLam_

- the previous check didn't catch bad cellZone names when running in
  parallel

- now export FOAM_CASE and FOAM_CASENAME environment variables from
  getRootCase() so they can be used immediately (eg, in decomposeParDict)
  instead of crashing!

  - could do with a 'nice' way to treat parent folders - constant, system, time dirs etc

- following Andy's idea to return values as label whenever possible
      eg, 1.2e6 -> 1200000
  but left it commented out

- avoid buffer overflow in ISstream::read(word&).
  Is the 'if (fail())' check itself actually in the correct place??

- other minor cosmetic changes