Move from sheet index -> sheet id about monorepo

[naterush]

How to actually move to sheet id: the sheet id should be an incrementing number, that goes up by one during every additional dataframe or every import.

This will allow us to preserve the replaying of analyses properly, while still having a unique ID per dataframe. Weird. It's not exactly an ID.

Or it's like: the IDs need to be consistently applied to the sheet number across repeats of the same analysis.

[naterush]

Oh. I just had an interesting idea:

1. All passed dataframes have sheet indexes < 0 (counting up to 0).
2. All imported/created data frames have indexes >= 0 (starting at 0).

This would do some pretty cool things to replaying the analysis, namely where we could avoid issues if users go from not passing a dataframe to passing a dataframe. The analysis would still replay properly. Cool!

[naterush]

NOTE: upgrading to this is a no-go though.. doh. We don't know how many dataframes user pass. We should put this metadata in, methinks!