Add de-rating op model

[paulf81]

Add de-rating op model

Adds an operation model to implement a simple de-rating strategy. Also adds a "mixed" operation model that essentially allows switching between various different operation models for different findices/turbine, which may be extended in future.

Shutting down turbines will be a natural use case for this, but this will appear in a separate PR (see also #687, #693)
Taking the steps from: https://github.com/orgs/NREL/projects/96?pane=issue&itemId=50191456

Steps:

• Create placeholder operating model that supports derating @misi9170
• Implement operating model that supports derating @paulf81
• Create tests to check operating model @paulf81, @misi9170
• Pass necessary control setpoints through from FlorisInterface() for derating @paulf81
• Pass necessary control setpoints through from FlorisInterface() for shutdown @misi9170 (see Add support to shut off turbines #693, may need reworking to align)
• Create example of derating, shutting turbines off, etc. @paulf81 , @misi9170

[misi9170]

I cleaned up a few small things; this looks good to me.

EDIT: making a minor change to the example based on @genevievestarke's suggestion.... Done.

[genevievestarke]

I like the example, looks good to me, too!