Swak

- Added pandas dataframe join wrapper

- Reinforced consistency in model dimension specification
- Pandas column(s) selector now also accept group-by objects
- Added pandas groupby and groupby.agg wrappers

- Fixed default separator in callback after every batch of model training

- Callback after every batch in model training now also get gradient norm
- Removed redundant argument to the YamlWriter
- Added JsonWriter and JsonReader

- Added a shorter log format
- Parent directory of log files will now be created if it does not exist
- PyTorch model training loop can now call multiple callbacks

- Renamed StdOutLogger to StdLogger and added option to log to stderr
- Renamed PassThroughStdOutLogger to PassThroughStdLogger
- Added option to log to stderr
- Added "close" method to batch callback to stay compatible with TensorBoard
- "close" method of batch callback is now called at the end of model training