added mh dir, moved mh support code there

also combined hmc code into one module
removed useless requires
enum-gibbs: add structural check, improve error message
added mixture transitions
add reinitialize method to mh-sampler

also catch up on exports
added example for hard-to-initialize models
revamp proposal code

proposals are now part of transitions (eg single-site)
adaptive single-site drift transition
turn proposals into objects, add feedback method
make proposal arg less verbose, more convenient

also clean up examples
fix dice example
add incremental initialization solution to dice example
add burn and thin args to generate-samples

also do SIR when given weighted sampler
add burn and thin args to sampler->*
update docs