Common interface for exporting data

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.