Added helix model and examples

[jfrederik-nrel]

• Added helix wake mixing model
• Added helix turbine power loss model
• Added examples to showcase new functionality

To do's:

• Format code @misi9170
• Write tests
• Format examples
• Add default tuning parameters for different turbines

[misi9170]

@jfrederik-nrel I've noted a handful of small updates (see below) that I think would clean this up.

Then, a slightly bigger question that we have discussed before would be to change the names of helix_amplitudes and helix_frequencies to be more generic so that they can be used in future wake mixing models (or, if even more generic, perhaps other ways of operating the turbine that are not necessarily for the purpose of wake mixing).

A couple of options would be:

1. wake_mixing_amplitude and wake_mixing_frequency
2. blade_pitch_amplitude and blade_pitch_frequency
3. pitch_induced_mixing_amplitude and pitch_induced_mixing_frequency

Others might have suggestions for this too!
Edit: @ejsimley suggested the following, which I like:
4. active_wake_mixing_amplitude and active_wake_mixing_frequency

Changing the names will be a little annoying as they appear in many places throughout Floris, but hopefully find and replace will be pretty effective once we decide on the names...

Edit: also after speaking with @ejsimley , it could be nice for the enable_helix_added_recovery flag on the floris input yamls to be slightly more generic---enable_active_wake_mixing could be an option here.

[misi9170]

Another note is that we'll also need to figure out how to handle the helix setpoints in the UncertainFlorisModel and ParallelFlorisModel. I'd say that for now, it's appropriate for those to simply raise NotImplementedErrors if helix setpoints are passed in (which I can implement), but it would be good to get others' opinions on that.

[rafmudaf]

This pull request was squashed into 9da4cbb.