Redesign

[MarcoGorelli]

• namespace.col: callable from DataFrame to Callable
• DataFrame.get_column: get (eager) column. Only supported for persisted dataframes
• DataFrame.take: remove? Or, remove unless persisted?
• sorted_indices / sort: only test for columns directly
• add testing utilities for columns
• remove parent_dataframe and persisted from Column and Scalar
• remove Column.persist and Scalar.persist

Would be nice to be able to do everything without reset_index