BUG: Waves object inconsistency throughout code

[cmichelenstrofer]

Describe the bug
The Waves xarray now has 3 dimensions: frequencies, directions, and realizations. But all the functions in WecOptTool expect only 2 dimensions. In the WecOptTool solver we itereate through realizations, selecting a subset of the wave xarray at a time, and then all functions work well on these 2-D xarray. Issues:

• The Wave module docstring still says the objects are 2D and does not reflect the addition of the realizations dimension.
• All functions throughout the code say the format of the waves argument is that from the waves module. That is not correct anymore. They accept a subset (single realization selection).

I am not sure what the best way to solve this is, without having to edit every single function that uses waves. Just documenting it for now.