Pipeline for processing inputs and outputs

- modify `Source` superclass so it can take a `input_preprocessing=[]` argument
- Each `Preprocessing` class has
  - `input` : the data source
  - `get_output_shape()` for method to allow `Source` to figure out the final output shape
  - `get_output()`
- pass input data through this pipeline, and modify n_inputs and seq_length.
