If all the cells are filled in on a combined round, complain! fixes #40.

I noticed that combined rounds don't check *which* solves were entered, all they require is that the very first time be entered. Here's the relevant code from WorkbookValidator:

```java
Long result;
if (round.isCombined() && resultIdx > 1) {
    result = CellParser.parseOptionalSingleTime(resultCell, resultFormat, event, aFormulaEvaluator);
}
else {
    result = CellParser.parseMandatorySingleTime(resultCell, resultFormat, event, aFormulaEvaluator);
}
results[resultIdx - 1] = result;
```

Ideally, we'd assert that all the incomplete rows have the same number of solves entered, and that there aren't any gaps in that range.