Add UNet processor

[sgreenbury]

Potentially add the ones reported in The Well.

[radka-j]

We agreed to just focus on the classic U-Net implementation (2015 paper, https://arxiv.org/pdf/1505.04597).

[radka-j]

See The Well implementation (adapted from PDEBench): https://github.com/PolymathicAI/the_well/blob/master/the_well/benchmark/models/unet_classic/__init__.py

[sgreenbury]

There is also an impl in Azula: https://azula.readthedocs.io/stable/api/azula.nn.unet.html#azula.nn.unet.UNet

[radka-j]

The Azula UNet is a more modern and complex architecture than the 2015 implementation everyone else uses as their baseline. But we could say we want to use that instead.