State space is not fully aligned with Julia iteration interface

[dylan-asmar]

The function state_from_index is implemented in states.jl. However, if we are following the conventions of the Julia iteration interface, we should extend Base.getindex.

Recommended functions to implement:

• Base.getindex(pomdp::TagPOMDP, si::Int) to replace state_from_index
• Base.getindex(pomdp::TagPOMDP, si_range::UnitRange{Int})
• Base.firstidex(pomdp::TagPOMDP)
• Base.lastindex(pomdp::TagPOMDP)