Eddy viscosity wake model

Introduces the eddy viscosity wake model, as described by Ainslie (1988) and extended by Gunn (2019).

The implementation comprises the following:

• Calculation of an initial velocity in the wake of a wind turbine
• Computation (via integrating an ODE) of the centerline velocities in the wind turbine wake for downstream locations
• Calculation of the wake width according to the usual Gaussian wake shape and momentum conservation
• Correction for the wake width via "streamtube expansion" when the wake of an upstream turbine encounters a downstream turbine
• Combination of wakes using the "sum of energy deficits" (Kuo et al., 2014)

These are implemented in the following locations:

• floris/core/wake_velocity/eddy_viscosity.py
• floris/core/wake_combination/soed.py
• floris/core/solver.py (streamtube_expansion_solver() and full_flow_streamtube_expansion_solver())

This pull request is currently a draft, while we work through final implementation steps and incorporate feedback.

TODO

• Implement eddy viscosity model main solve
• Test main solve
• Implement eddy viscosity "full flow" solve for visualization and flow sampling
• Include wake deflection model (currently raises NotImplementedError with nonzero yaw angles)
• Add initial set of examples
• Handle warnings raised
• Incorporate feedback and comments on implementation, in particular:
• where streamtube expansion should be applied
• which turbulence intensity (either atmospheric or wake-induced) should be used to initialize eddy viscosity model for downstream turbines
• whether and how to use the near-wake correction filter
• Clean up and finalize examples
• Add documentation