Add support for results version overrides

[geoffxy]

Right now, we use an involved process to select the version to use for a run_experiment() task. This process is a good default policy. However sometimes we want to override it (e.g., use a prior version of the results).

Rough idea:

• Add another table to version_index that stores an override for task identifiers
• When processing dependencies and setting COND_DEPS, consult the override table before running our version resolution logic