Native pickling, updated opt_tools

Closes #40 (closed). Pickle works out of the box for FrankaData and DynSliceDataset. Not DynSliceDataloader, as this has a generator for the iterator which wont pickle.

Changelog:

• Patched opt_tools where we dont use inheritance for (Traj, Batch, TrajBatch) but just a metadata field. Moved lots that's not needed for you guys here to OptVarSet. A few new helper fns for shapes and casting in opt_tools.
• moved tests from foot of dyn_slice_dataset to tests/test_dynslice.py and tests/test_data_load for the fake/real data respectively
• python -m pytest results in tests.{training_real_data, test_flattening, test_data_load, test_dyn_slice} working

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Questions:

• tests/
  • loading_data.py is not a test? Also it's loading models?
  • do we have a test for pickling/unpickling models?
• trained_model_tests are not in .gitignore intentionally?
• create_dataloader/create_data is dead? Or at most oneliners that we can merge?
• utils/validation_fn::plot_* are commented out?