Refactor src/stan/gm/command.hpp: debugging is difficult as is.

[syclik]

Issues like #379 are masked by the structure of src/stan/gm/command.hpp. Here are some things that should be done:

• consistent use of return codes. Use the values in src/stan/gm/error_codes.hpp
• remove switch in command()
  • use of sampler_select is opaque. Too hard to see which sampler is actually being called.
• double check all exit codes
  • currently, there are no tests for how this exits

We should shift focus of this code and favor clarity and maintainability. This code should be Stan's interface to be utilized by CmdStan, RStan, and PyStan. (CmdStan and RStan currently does this. I'm not sure about PyStan.)

[syclik]

#392 is a similar issue and deals with the switch statement.

[betanalpha]

Switch removed in https://github.com/stan-dev/stan/tree/feature/issue-1361-writer_callback.

[syclik]

We'll fix this with #1361.

[syclik]

I'm closing this issue. It's being addressed in other issues and the file has moved to CmdStan.