Configuration option for various decoders and structure of the decoder types

[Krastanov]

In #212 we introduced many decoders (as interfaces to other libraries), but we do not support all the available functionality. This should be added:

• all config options for PyMatchingDecoder

• all config options for PyBeliefPropDecoder

• add PyBeliefPropOSDDecoder

• all config options for the LDPCDecoders.jl

  • belief prop
  • belief prop with osd
  • iterative small set flip

• Relatedly, we should better organize the structures representing these decoders. We should have separate config and cache inner types. E.g. something like:

struct BeliefDecoder...
    config::BeliefDecoderConfig
    cache::BeliefDecoderCache
    ...
end

• These types should be concrete.