Natural frequency in multiple DOFs

[rgcoe]

Similarly to #69, we need to verify the functionality of our natural_frequency method and function in multiple degrees of freedom.

https://github.com/SNL-WaterPower/WecOptTool/blob/7d9bbc70f7cac23b468d5b073d6da9727c68be9e/wecopttool/core.py#L1105-L1127

[cmichelenstrofer]

This didn't make it into v2.0. v2.1 is planned mostly for CI/Testing. Will leave this open as a good first issue.

[rgcoe]

Note that both tutorial_3_LUPA.ipynb and #214 present good opportunities to test this functionality.

[jtgrasb]

This would be a pretty easy thing to add back in to utilities.py if we want to.