Replace sampletype with Random.gentype

[zsunberg]

just noticed that this is the standard

[lassepe]

To my understanding, Random.gentype has not been documented on purpose, because it is unclear at this point whether it should be part of the official API. In fact, I can't find any documentation of it in the v1.2 docs. It looks like this is still in some transient phase and people are leaning toward using eltype instead.

Distributions.jl suggests that people should use eltype as well; quote:

By default, Discrete sampleables have support of type Int while Continuous sampleables have support of type Float64. If this assumption does not hold for your new distribution or sampler, or its ValueSupport is neither Discrete nor Continuous, you should implement the eltype method in addition to the other methods listed below.

Maybe we should suggest to implement the Distributions.jl Sampleable interface?

[zsunberg]

Actually, now I am leaning towards just getting rid of it.

It doesn't seem like we actually use it anywhere: https://github.com/search?p=2&q=org%3AJuliaPOMDP+sampletype&type=Code, and it is probably actually safer to let the compiler infer it than to make people write it.