Allow observed data to inherit dims

[ColCarroll]

Using a pattern like

data = az.pystan_to_inference_data(fit=fit, 
                                observed_data=['y'], 
                                coords={'school': schools},
                                dims={'theta': ['school'], 'y': ['school']})

throws an exception without this change.

[canyon289]

LGTM waiting for tests to pass

[ColCarroll]

hold on for a sec - adding this case to the test suite. might have to clear the cache on travis, so it will take a while.

[ahartikainen]

LGTM.

This is assuming we are changing pccplot and observed data to match ppsample.

[ColCarroll]

Just deleted caches and restarted travis. This is good to merge if tests pass (also fine waiting for #248 since mine is easier to rebase).