add n_draws and t_sampling report to smc

[aloctavodia]

Adds trace.report_n_draw mainly to allow ArviZ to work with a trace from SMC, also adds _t_sampling, because having access to the sampling time is nice :-)

Fixes #3928

[michaelosthege]

@aloctavodia I'm sorry, my last edit broke your PR. I tried to set _n_tune, but used a n_tune variable that does not exist.
I just checked it out, but I don't understand how the tuning works, so I don't know how to fix it =(

[aloctavodia]

@michaelosthege @AlexAndorra Tests are passing for SMC and failing for other samplers

[AlexAndorra]

@aloctavodia, I think the tests are failing because of the bug in io_pymc in ArviZ. This is fixed on master. Do we wait for the ArviZ bugfix-release to relaunch the tests?

[OriolAbril]

Bugfig release for ArviZ is out, however, other samplers failing is probably because they do not have n_tune,... in report. ArviZ won't fail anymore, but save warmup won't work for these samplers

[AlexAndorra]

Yeah, seems like the problem is a mix of n_tune absent and n_draws being None when the trace is sliced manually -- Michael opened a PR about this.