New features

• Pre-emphasis filter and loss (#42)
• WaveNet and CatWaveNet models (#43)

Bug fixes and performance improvements

• Fix bug exporting parametric models (#41)
• Remove redundant "nx" in data set config; let the model provide it w/ its receptive field.
• Faster data indexing in ConcatDatasets with many data sets (#45)
• Miscellaneous improvements to the LSTM model.
• Improvements to the Colab notebook.