Create InferenceData from trace and sampled posterior prediction

[bridgeland]

@OriolAbril

with model:
    idata = pm.sample(...)
...
with model:
    ppc = pm.sample_posterior_predictive(...)
    idata_aux = az.from_pymc3(posterior_predictive=ppc)
idata.extend(idata_aux)

Note that idata.extend() requires arviz v0.10.0