Add ability to concatenate 2 IDataViews.

I have:

- IDataView trainingData
- IDataView testData

I want to combine trainingData and testData
`IDataView combinedData = trainingData + testData;`

Finally, I want to retrain the model with it.
**Q1. Is there are way to combine multiple IDataViews without converting to DataFrame?**

**Q2. What is the syntax for combining IDataViews converted as DataFrame. I am trying to do something like this:**
```
DataFrame dfTrain = trainingData.ToDataFrame(-1);
DataFrame df2 = testdata.ToDataFrame(-1);
df.Add(df2.Rows);
```

The [sample in machinelearning-samples](https://github.com/dotnet/machinelearning-samples/blob/ba98d520e67a2840a3c50410885a24f361632567/samples/csharp/getting-started/AdvancedExperiment_AutoML/AdvancedTaxiFarePrediction/Program.cs#L194) re-reads the data from a file source. However, it the data might not always be in a file, or the big may be very big