Simplify generator spec

[kiritofeng]

The current generator spec consists of several hacks to handle previous unintended use cases (primarily that if the generator crashed, the input/output files would be passed directly to the program).

This has led to an opaque generator spec that is not documented and has several counterintuitive behaviours.
For example, specifying input_file and no output_file will run the generator with the input file passed to the generator via stdin, but if an output file is then specified, the generator will be totally bypassed.

I propose rewriting the spec for generator behaviour to the following. For an init.yml with a valid generator node:

• If generator_args are specified, the generator is run with arguments (and no file input). Including input_file or generator_input_file will result in an InvalidInitException.
  • If output_file is included, it will be passed as the judge output.
• Otherwise:
  • If generator_input_file is specified, the generator is fed the input file. Including input_file will result in an InvalidInitException.
    • If output_file is included, it will be passed as the judge output.
  • Otherwise:
    • If input_file is specified, the generator is ignored for that test case, and the files are passed directly to the submission
    • Otherwise, the generator is run with empty stdin, and output_file is passed as judge output if it exists.

This should hopefully result in more intuitive and well-documented behaviour.