Useful methods for <:SSA types

[slwu89]

Address #37 and #36, for all of the "single samplers" adds methods getindex, length, keys, and keytype (see src/sample/interface.jl), and added tests.

Additionally:

1. Fixed the typing for abstract methods in src/sample/interface.jl
2. Changed the MutableBinaryHeap to a MutableBinaryMinHeap in the combined next reaction sampler
3. @adolgert I am not sure what Base.getindex should return for the DirectCall sampler (see src/sample/direct.jl), I thought the leaf nodes of the prefix tree would be the distribution rate parameter but it does not seem to be the case. Can you help me take a look at that? That's why the newly added test in test_direct.jl is commented out. I also added Base.getindex methods to some of the trees in src/prefixsearch. This also raises the general question of what it should return as in some cases we store distributions and in other cases we store sampled times.

[adolgert]

Ah, that's right. It isn't the rate. The data structure is a prefix sum tree, aka Fenwick tree. It stores partial sums at nodes, so you have to back that out to get the hazard for an Exponential distribution. I'll take a look.

[adolgert]

Hey @slwu89, I think I fixed the problem, but I don't see the test that's commented out. I pushed to the branch with the fix. It's not a Fenwick tree. It's a simple binary tree, stored as a one-based array.

[slwu89]

thanks @adolgert! I'll update from my end and then ping you when I think we're ready for the final review before merge.

[slwu89]

Ok @adolgert this is ready for your review.