Add override flag

[b-fenelon]

Not an urgent feature

Following Jon's recommendation to #45, created an '--override' flag that accepts a YAML-like input. This adds a replace method to EvaluationCard that enables programmatic changes to multiple symbol values, which is useful for testing. This only allows value (must preserve type) and sweep replacements, limiting the changes possible without updating the card itself.

I am currently omitting a README section because I don't think we should recommended this approach over explicitly defining a new card. Once we have a standard output format, the results should capture the necessary details anyways to recreate the card with overrides.

Example usage:
magnet evaluate magnet/cards/llama.yaml --override "{helm_runs_path: ./other-data-path/benchmark_output, threshold: 0.01}"

[dmjoy]

LGTM

[b-fenelon]

@Erotemic Not a critical feature. Let me know if you have any thoughts (mostly follows your example from #45)

[Erotemic]

This is the only thing that I see that raises a red flag. The flag is doing an "override", but this is extending an existing list and there is no way to tell it to use a new list instead. If this is needed there is some trickery that can be done. You could set the value to be a dictionary with one key like __extend__: [1, 2, 3] and that very particularly special case could be interpreted as wanting to extend the sweep list, and it would be explicit in the CLI (at least if you had knowledge of the behavior, but otherwise it looks weird enough to at least cue someone into some sort of special behavior happening)

[b-fenelon]

I'm in favor of dropping the append. This flag is mostly for debugging and only works for python symbols (i.e. not kwdagger components). I originally didn't want to override lists to dissuade users from using '--override' in place of creating a new card. But, I agree that it's a weird behavior. If users want more flexible sweep handling, we can revisit the solution you give here

[dmjoy]

@Erotemic it looks like Ben addressed the issue here, are we good to merge?

[Erotemic]

Yeah, go for it.