`read_pomdp` doesn't appear to fully implement the `.pomdp` spec.

The spec outlines that one can use `start:` to designate what would be `initialstate`. Additionally, there are a variety of keywords for `start` that can be used to modify the distribution of `initialstate`.

I need to dig further into this, and I'll probably prep some fixes for it, but figured I'd flag it now.

Start at the read_pomdp entry point and compare its handling of .pomdp input with the referenced specification. Check how start: and the available start keywords should map to the initialstate distribution. Done means the supported forms are implemented according to the spec and representative inputs are handled correctly.

Written by the indexing model from the issue text.