Improve reproducibility of preprocessing; add ALCF documentation

[felker]

• Prefixed preprocess shot .npz files with machine name. Closes #45.

• Preprocessing and normalization diagnostics:

  • Write out [omit] every time a shot is excluded from either procedure
  • Print totals of non-/disruptive shots after preprocessing, alongside split into train/test/dev totals for each class.

• Remove redundant "data" token from shot list variable names in conf_parser.py

• Add ALCF documentation

• Use 2 space indentation in all YAML

• Delete single-GPU runner.py

• Add tcn.py file (c522d3be7a53638676d87af97960219816ae6679) missing from #50.

• Add ONNX writer using https://github.com/onnx/keras-onnx

• Start encapsulating module, pip, and conda dependencies in platform-dependent files in new directory envs/. Closes #47.

• [ ] Add YAML linter

• [ ] Update ALCF Theta documentation

• [x] Make ONNX writer optional; builder.py should not fail due to import onnx or import keras2onnx