Add estimators module

[misi9170]

Add top-level module estimators that can be assigned to certain controllers where an estimation step is needed/relevant. Main example at this point is spatial averaging/consensus for enhanced wind farm yaw control.

• Establish whoc/estimators
• Establish abstract EstimatorBase, similar to ControllerBase, that enforces a compute_estimates method
• Put in a basic estimator for wind farm control purposes (e.g. uses average measurements from freestream turbines)
• Possibly add "null" estimator that passes through measurements unchanged (may not be needed)
• Add tests
• Incorporate in relevant examples
• Update documentation