Common interface for exporting data

[kdheepak]

Currently, there's to_dataframe but it does not cover all possible objects in OpenDSS. Also, the interface of Bus is different from Line and Transformer. This is currently implemented as such because of the way OpenDSS stores information regarding these objects, but a good Pythonic interface could abstract that away. It would be "a nice to have feature" to have a common way of exporting all information of all objects of a certain type from OpenDSS.