Expose distributions at root level of pymc_experimental

Also `marginalize`

Author: ricardoV94

pymc_experimental/__init__.py

@@ -23,7 +23,8 @@
         handler = logging.StreamHandler()
         _log.addHandler(handler)
 
-from pymc_experimental import distributions, gp, statespace, utils
+from pymc_experimental import gp, statespace, utils
+from pymc_experimental.distributions import *
 from pymc_experimental.inference.fit import fit
-from pymc_experimental.model.marginal_model import MarginalModel
+from pymc_experimental.model.marginal_model import MarginalModel, marginalize
 from pymc_experimental.model.model_api import as_model