Move away from `Base.write(::IO, ::POMDP)`

I don't think specializing `Base.write` for `POMDP` is the correct way to implement the functionality in this package.

[The docstring for `write`](https://docs.julialang.org/en/v1/base/io-network/#Base.write) says "Write the canonical binary representation of a value to the given I/O stream or file." I don't think the pomdp file format is the canonical binary representation they are thinking of based on the rest of that documentation (it talks about endianness). `Base.print` may be more appropriate, but still probably not what we want.

I think we should look at how DataFrames are written to csv files or how excel files are read in for some inspiration. Or maybe if there is a package that saves neural networks to bson?