Improve vessel classes

[redhog]

I have evaluated the current vessel classes and how well they map to classes of actual vessel behavior by clustering an embedding exported from the neural net.

Embedding export code: #166
Clustering evaluation: https://github.com/GlobalFishingWatch/vessels_gear_embedding/blob/master/vessels_gear_embedding.ipynb

According to these results, the purse seiner class is actually made up of two classes that have vastly different size and quite different gear, possibly coastal and oceanic purse seiners.

Please investigate this further, and let's see if we can create a training data set for this!

ping @bitsofbits @pwoods25443