ENH: code reduction, improvements for expressions

- literal lookups only for expression strings

- code reduction for setExprFields.

- changed keyword "condition" to "fieldMask" (option -field-mask).
  This is a better description of its purpose and avoids possible
  naming ambiguities with functionObject triggers (for example)
  if we apply similar syntax elsewhere.

BUG: erroneous check in volumeExpr::parseDriver::isResultType()

- not triggered since this method is not used anywhere
  (may remove in future version)