DSS commands in the API

[PMeira]

There are maybe hundreds of DSS commands that are not exposed as explicit API functions. Besides the long-term goal of exposing function sets for each DSS component, we also need to expose the commands. E.g. save circuit (dss-extensions/OpenDSSDirect.py#89) doesn't seen to be exposed yet.

The APIs are the initial documentation most users find, so expanding them to include these will certainly help users in general.

[PMeira]

I'm working on this, hopefully will be done in a week or so. This became a blocker for other features and the documentation, hence the new priority.

The changes should address most of #56 too (complementing #91), part of #45 and #96.