Model ``Contingency``

[jinningwang]

Is your feature request related to a problem? Please describe.

Describe the solution you'd like
New model, Contingency, provides a contingency list for N-1 secured dispatch.

The model is expected to store the indexes of devices that can be outaged.

It has two parameters: model, which is the model or group name of the device, and dev, which is the list of device indexes.

Describe alternatives you've considered

Additional context

[jinningwang]

Similarly, we should introduce a Monitor model to represent the lines of interest.

With this model in place, only the monitored lines will have line flow limit constraints in effect.

[jinningwang]

In practice, Contingency and Monitor are paired. For example, for one individual contingency, there will be several monitored lines.