JIT parameterization using protobuf

We already a number of options to pass to JIT module, and plain to have more.
This is a good time to make transition to protobuf based parameterization.

- Begin with `src/generate_cxx_code.cpp`
- Add a message schema to `src/proto/wcs_params.proto`
- Modify the examples under `src/proto/test*`
- Add a parameter structure under `src/params`
- Add functions to populate the structure in `src/proto/wcs_paraps.cpp` from user-provided prototext
- Integrate the metho into `src/generate_cxx_code.cpp` (look at the example in `src/ssa_partition.cpp`
- Apply it to `src/reaction_network/network.cpp`