Changable TSDataset attributes

[d-a-bunin]

Description

Currently, in TSDataset we store public attributes:

• df
• raw_df
• df_exog
• known_future
• freq
• hierarchical_structure
• current_df_level
• current_df_exog_level

It make these attributes look available for writing after the init, but in most cases changing them after init could break the consistency of the class.

We should somehow prohibit changing them in most cases and make them changable only in cases when we are sure about the consistency.

Thoughts

We could wrap these attributes into property.

Related

• Changable `BaseMixin` attributes #418
• Prohibit different segments to have different set of features in `TSDataset` #433