Dataframes of generic types of network elements (identifiables, injections, branches, terminals)

[sylvlecl]

• Do you want to request a feature or report a bug?

Feature

• What is the current behavior?

When we want to get for example a list of all injections, we need to call multiple methods and concatenate the resulting dataframes. For example, it's pretty hard to get the list of all elements connected to a specific bus.

• What is the expected behavior?

Instead the API could provide native methods to get common information of a generic type:

• get_identifiables: all identifiables, with only their id
• get_injections: all injections with voltage_level_id, bus_id
• get_branches: all branches with voltage_level1_id, bus1_id, voltage_level2_id, bus2_id
• get_terminals: the list of all terminals, described by their element_id, element_side, voltage_level_id, bus_id

In particular, the get_terminals method provides a practical view to the users of those objects which are very important in the java implementation.

[sylvlecl]

I would appreciate some opinion about this API evolution before merging the related PR (thanks @AnkurArohi for the thumbs up ^^ ), in particular about the "terminals" part: @annetill @geofjamg