assign codes to constants tokens as soon as possible

findCode() assigns code for expected data type
getOperand() assigns code when specific data type is requested
processInternalFunction() assigns code when expected data type is number
getExpression() does not need to force constants to expected data type
outputAssignCodes() no longer assigns constant token codes
token type changed to Constant for constant codes
debugName() modified to catch invalid code
token text() modified to output code text (with index) or '?' in front
  of constants with invalid code (not with index)