Add WEC-Sim power performance example

[akeeste]

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This PR adds an example that uses MHKiT and WEC-Sim together to do a power performance assessment:

• utilize resource characterization in PacWave notebook
• write MCR cases for WEC-Sim
• load WEC performance
• assess WEC performance

Right now I represent the resource using the 32-cluster results in the PacWave notebook and irregular waves in WEC-Sim. I was considering writing all 2000+ wave cases in that example to a WEC-Sim MCR file so that we can demonstrate mhkit.wave.performance.power_performance_workflow() but decided it would be too expensive, even if regular waves were used in WEC-Sim.

I also have a half-finished example that shows how to write a directional spectra (like in directional_waves.ipynb) for WEC-Sim. But the WEC-Sim 'full directional spectra" feature needs documentation and an example first.

The PR also includes a function to make the xarray representation of WEC-Sim data a lot more useful