Add transactive orderbook

[dchassin]

This PR addresses the need for an order book implementation of a Transactive system.

Current issues

1. Documentation is not completed. See iPython notebook.
2. Implementation is not completed.

• Buy and sell limit, same duration
• Buy and sell market, same duration
• Buy and sell, different duration
• Ramping examples

1. Validation is not completed.

Code changes

1. Orderbook class has been added to the transactive module.

Documentation changes

• Added transactive module documentation to wiki main page.
• Added orderbook to wiki.

Test and Validation Notes

None

[dchassin]

Closing due to lack of progress. Work will continue on dchassin/gridlabd.