Legacy model weights import

[talmo]

Core SLEAP models are saved as Keras HDF5 files, which makes them fairly easily readable without actually depending on TensorFlow or Keras.

The task would be to understand the structure of those HDF5 files, load up the weights, possibly transpose the ordering to match the kernel ordering in PyTorch, and assign those values to the appropriate nn.modules.

See also: #63 which is a pre-requisite so that we can instantiate the models as PyTorch nn.modules in the first place.