Simple IDataView Sample

[TomFinley]

Though we have many methods or samples to create data views from existing structures, like files, databases, enumerables, lists, and so on, we can't possibly handle everything, so a sample as to how to create a data view may be desirable.

In this first simple sample, my goal would be to show a fairly minimal IDataView implementation but from absolute scratch, one that is very simple without being, I hope, simplistic. I might however stick to scalar inputs for that first simple example, and definitely avoid anything having to do with enabling shuffling or parallel cursor evaluation.

Related to #1209, but given that this does not describe a sample for anything directly related to MLContext, it was thought best to have a separate issue for it.