Add missing toJSON methods

[sylwiabr]

Summary

toJSON method is called whenever Luna Studio is trying to show visualization for a node. Missing toJSON methods causing errors on a graph.

The reproduction steps for above:

1. read the file i.e. the one from Technical Analysis (welcome screen example)
2. get single column from the data frame (column "Date")
3. take field from the column header
4. get type from the Field

Value

Better visualizations on a graph.

Specification

The toJSON methods should be added to all Classes in Dataframes like:

• Field
• TypedColumn
• DataType
• Schema
• StringType, Int32Type etc.

Acceptance Criteria & Test Cases

The nodes with this types should have proper and informative visualizations