Fixes a crash when datasource is empty and makes datasource rows public

Summary:

• Fixes crash when datasource is empty
• Demo project updated to show an empty state
• rows property is now available allowing you to access the current datasource as it is - maintained by SwiftDataTable.
• There's a new method which allows you to fetch the data value type for a given index path - see technical information for more details.

Technical:

• There is a new method added allowing you to fetch a data value type for a given index path:

public func data(for indexPath: IndexPath) -> DataTableValueType

This method can be used as a convenience method or you can use the rows property to access the multi-dimensional array manually.