`min_train_window`, `quantile_by_key` args are not used

Grepping around, @dshemetov and I were unable to see `min_train_window` or `quantile_by_key` actually used by `arx_forecaster`, `arx_classifier`, `flatline_forecaster`, or any other forecaster I missed. We did find:
- There might be a PR for the `min_train_window` part; see comment [here](https://github.com/cmu-delphi/epipredict/blob/9a804303d97aa9d3de1323c66a52797acce86483/R/arx_forecaster.R#L56).
- `quantile_by_key` docs reference `layer_residual_quantiles()`' `by_key` parameter, but I can't see it actually being forwarded along.