Add back in CRP, NIGN, DPMM example
I am removing these temporarily, in order to allow the major interface update of Gen to be merged into master, without having to patch up that feature. The interface update is being rushed because some code required for AAAI depends on it. The ExchangeableJointGenerator, CRP, NIGN, and DPMM examples aren't critical for any current projects. They should however, be added back in, as they are an excellent fit for Gen, and the new simulate! and regenerate! interface in particular.
Old versions of ExchangeableJointGenerator, CRP, NIGN, and associated unit tests can be found in this old commit on master branch:
https://github.com/probcomp/Gen.jl/commit/d41ef9537e0216d28e93aae92906d01a72ec11e6
Old version of DPMM examples is in gen-examples at https://github.com/probcomp/gen-examples/commit/1dac5d7f9dd4eb3c3ec72f2953f6fc77d6e786c6