TTreeReaderValue<double> reads a double[] branch by only accessing the first element with no warning

If a TTree contains a branch "arr" that is a fixed or variable size array of, say, doubles, TTreeReaderValue<double> reads it without problems, but only accesses the first element.

We should probably error out instead, suggesting to use a TTreeReaderArray<double>.

Issue first found here, in the context of a TDataFrame analysis.