update todo
export with-zone, transition constructors
added sampler->means+covariance
update erps
fixed Denergy for gamma
removed more contract result checks

The point is to make sure that sample is actually called
in tail position wrt the use of the ERP in the program.
fix uniform-dist and uniform args
removed broken observe-at for rejection-sampler

The rejection-sampling factor M must cover all possible
traces, not just the current trace (and in particular,
the current dist params).
Merge remote-tracking branch 'upstream/master' into akliger/to-upstream
Merge branch 'akliger/to-upstream' into wip/hmc-improve
hmc: If leapfrog moves into a region with likelihood 0, reject the entire leapfrog big step.
Linear regression using Hamiltonian Monte Carlo.