BF: fix incorrect parsing for single sample (ie, optimizing)
BF: fixed failure to return default params w/out version input
ENH: Use shallow copies of StanModels to maintain independence between StanFits

stan()
 - use shallow copies of model objects
 - allow StanModel instance as input as another way to avoid compilation

StanFit
 - print() works for optimizing
 - some warnings when incomplete chains are read

StanModel
 - BF: fix bug where model_home was getting reassigned to working_dir
 - implement shallow copy()