Add Empirical Gauss wake model for fitting SCADA data

[misi9170]

This pull request is to incorporate the new Empirical Gaussian wake model (working name Geometric Model).

This PR includes:

• The new parametrization of the Gaussian wake model designed to be simpler to tune and more flexible for fitting observed wind farm data, consisting of
  • a new velocity deficit model
  • a new velocity deflection model
  • a new "wake induced mixing" model to replace wake-added turbulence
• Vertical wake deflections due to rotor tilt
• Mirror wakes to address stumpiness and ground effects

TO DO before merge:

• Add example to demonstrate usage of empirical_gauss model
• Check default tuning, ensure defaults match the example input file emgauss.yaml
• Check documentation in docs/empirical_gauss_model.md and develop docstrings for new functions, classes, and methods
• Check/update current tests,
• Develop regression tests

NOT included in this PR, but for possible future developments:

• Implement unit tests for lower-level functions as part of the simulation tools, e.g. sigmoid_integral
• Profile code and look for speed-up opportunities

[rafmudaf]

@misi9170 I made a few syntax and style changes in the last few commits. There's nothing fundamentally different and the tests pass, but I think you should review them just in case. Otherwise, this looks great. Thanks for making it easy to review and good work!

[misi9170]

@misi9170 I made a few syntax and style changes in the last few commits. There's nothing fundamentally different and the tests pass, but I think you should review them just in case. Otherwise, this looks great. Thanks for making it easy to review and good work!

@rafmudaf Thanks! I just looked at the diffs for the last five or so of your commits and everything looks good to me.