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.