Enable vars argument in misc._print.stanfit

Removed `if-else` block and moved the `pars` handling to `_summary` function.
Now one can print summary for chosen pars. 

`print(pystan.misc._print_stanfit(fit, pars=['alpha', 'beta[0]', 'sigma'], probs=(0.025, 0.50, 0.975), digits_summary=3))`

Author: ahartikainen

pystan/misc.py

@@ -56,13 +56,6 @@ def _print_stanfit(fit, pars=None, probs=(0.025, 0.25, 0.5, 0.75, 0.975), digits
                    "sampling is not conducted.".format(fit.model_name)
         elif fit.mode == 2:
             return "Stan model '{}' does not contain samples.".format(fit.model_name)
-        if pars is None:
-            pars = fit.sim['pars_oi']
-            fnames = fit.sim['fnames_oi']
-        else:
-            # FIXME: does this case ever occur?
-            # need a way of getting fnames matching specified pars
-            raise NotImplementedError
 
         n_kept = [s - w for s, w in zip(fit.sim['n_save'], fit.sim['warmup2'])]
         header = "Inference for Stan model: {}.\n".format(fit.model_name)