add Naturals0
add drv and PoissonDistribution
solve, sample, docstring in discrete
add Geometric Distribution
density -> distribution in frv.SingleFinitePSpace
refactor symbol, value, distribution to SinglePS
move _value_check to rv
single discrete pspace
hack compute_density in discrete
discrete distributions act like Lambdas
docstrings for discrete distributions
import stats/discrte into __init__
evaluate=False in discrete
test_args for discrete
next(...) -> .next() in test_fancysets
remove excess whitespace
import drv_types to __init__
fix doctest in Poisson
SinglePSpace Symbol creation uses Symbol
symbol -> Symbol in SinglePSpace constructor