change repo spec to support multi-run experiments

[vladvelici]

The issue mentioned in the readme file where experiments are currently defined by only one run. this shouldn't be the case and more runs must be supported.

this issue is for tracking the progress in making a repo spec change to add support for multiple runs per experiment

I think the best example use case is when you have separated training and evaluation scripts - all the git, host, env vars, os args tracking should happen per-run but parsed hyper-parameters should remain per-experiment.

I'm looking for a design that keeps the simplicity of a single meta.json, maybe with an extra file per run and moving all the env, args, git and host stuff out of meta? The final format to be decided and implemented.