As a user I would like the abilityto add calculated columns to Vuu (using Antlr4 grammar)

[chrisjstevo]

Feature Request

As a user I would like the ability to create columns dynamically on my viewport from configuration, and for that to be persisted with the viewport in the json description.

The flow would work a little like this:

1. I create a new viewport on an existing table
2. I add a new calculated column to that viewport
3. The calculated column definition is handed to the server in the format something like CalculatedColumn(name: String, DataType: Int, calculation: String)
4. On receipt of this definition from the client (in a CreateViewport or ChangeViewport message), the server would parse the calculated column and add it to the viewport.
5. When the row is subsequently realised (i.e. the key in the viewport is translated into a full row and the JSON message is generated, the calculated column would be evaluated.

Description of Problem:

As a user I want to be able to create custom column values from the data I generate and for them to be returned in a viewport.

[chrisjstevo]

Items still to check:

• nulls in calculation, how should they be handled?
• calculations of calculations (etc)
• trees of calculations
• calculations in trees