[PATCH] Make df.data_type a property that accesses df._data_type

[jinimukh]

Before, the user could change df.data_type by setting it to some other dictionary. This would benefit from being a "protected variable". Thus, df.data_type is a property getter for df._data_type.

Also, df.data_type only returned a non-null dictionary if df._repr_html() was called before. Now, the getter calls df.maintain_metadata() if df._data_type is null.

Changed:

• changed metadata attribute df.data_type to df._data_type
• added a getter that calls df.maintain_metadata if df._data_type is NULL

[dorisjlee]

This looks good @jinimukh, could you resolve the comment and branch conflict before we merge? Thanks!