Suggestion related to Working with Columns

[ddobric]

Right now we use Column attribute to decorate how to read columns. Additionally we mark column with text "Label" to indicate that it is a label.

If label column is missing ore not correctly named we get: "InvalidOperationException: Column 'Label' not found"

Suggestion:
It might be better to use dedicated attributes like FeatureColumn and LabelColumn instead of freely written text. It is a statically typed and more .NET (C#) agnostic.

[sfilipi]

@ddobric your feedback is incorporated in the static api described in #474, #581 and the cookbook.

Closing this since the discussion of the same topic in the other Issues is brader.