Generalize DBMSticher to  N RBMs, rename it RBMSticher
Revert CD's positive phase to be like VariationalPCD's positive phase
Fix a bug in TransformerDataset

The bug prevented TransformerDataset chaining since a TransformerDataset has no
'y' attribute, so if a TransformerDataset has another TransformerDataset as its
raw_dataset, its has_target() method returns by calling its raw_dataset's
has_target() method.
Fix a bug in Softmax's make_symbolic_state method
Restric DBMSampler to DBMs with one hidden layer (for now)