Update the way cuts are introduced inside `TRestDataSet` so that we can use `TRestCut`.

[jgalan]

The following issue should introduce the use of TRestCut inside the TRestDataSet definition.

This way we could define our config file as:

<TRestDataSet ...>
....
<TRestCut>
  <cut name="cc1" value="XX>10 AND XX<90"/>
  <cut name="cc2" value="sAna_ThresholdIntegral<100e3"/>
</TRestCut>
...

</TRestDataSet>