Distribution add ons

[markkho]

Additional things for FiniteDistribution:

• factor method that weights outcomes (a la webppl)
• then, normalize, from_pairs, join methods (see

  msdm/msdm/domains/gridmdp/windygridworld.py

  Line 10 in a88a5a8

  class DictDistribution(DictDistribution):

  )
• syntactic sugar with ~ for sampling

[markkho]

I've started making some of these additions in this pull request #57 (comment)