POMDPLinter Functionality

[zsunberg]

Someday someone (like me) might want to make a linter for POMDP problems. This issue will be a checklist for functionality.

• transition and observation return distributions, not states or observations #390
• functions are typestable (Performance help script #159)
• isterminal is consistent with transitions and/or solvers check for isterminal (terminal support POMDPFiles.jl#5)

[zsunberg]

• states hash correctly and are completely immutable