BUG: incorrect boundary type for expressions variables (fixes #1889)

- had calculated boundaries (default) for the evaluated variables,
  which meant they retained their initialized values (usually Zero).

  This normally goes unnoticed, since the boundary values are largely
  irrelevant in the volField expressions. However, when applying
  functions that balk at a zero value - eg, log() - this raises a
  floating point exception.

  These boundary should be zeroGradient, since the evaluated variables
  correspond to the internalField only. Could continue to use
  calculated, but then need to set the calculated boundary values from
  the patch internal field manually.