Refactor SMC and properly compute marginal likelihood (#3124)

* [WIP] refactor SMC and properly compute marginal likelihood

* generalize to variables with shape > 1

* fix tests

* use sample_prior_predictive to generate initial population

* fix tests, for real

* make test more robust, just in case

* add experimental warning message add change to release note

Author: aloctavodia

RELEASE-NOTES.md

@@ -1,9 +1,11 @@
 # Release Notes
 
 
-## PyMC3 3.6
+## PyMC3 3.6 (unreleased)
 
 - Renamed `sample_ppc()` and `sample_ppc_w()` to `sample_posterior_predictive()` and `sample_posterior_predictive_w()`, respectively.
+- Refactor SMC and properly compute marginal likelihood (#3124)
+
 
 ## PyMC 3.5 (July 21 2018)
 
@@ -440,3 +442,4 @@ Thus, Thomas, Chris and I are pleased to announce that PyMC3 is now in Beta.
 * maahnman <[email protected]>
 * paul sorenson <[email protected]>
 * zenourn <[email protected]>
+