Add functionality to log values of buffers in a model (#220)

* In all logger types (PythonLogger, TensorBoardLogger, CSVLogger)
* Exact behavior varies per logger type and documented in the code.
* To enable in CSVLogger, changed its API to take a file name prefix
  (optionally empty) instead of the full name, and use a hard-coded 
  name for logging weights sparsity.
* Also fixed signature of log_training_progress in base DataLogger
  class to match the signature used in the sub-classes.