Proposal: Remove assetId as requirement and make grouping optional

[jayqi]

Right now append_slope_features requires a column named "assetId" which it will use to group observations for the purpose of calculating stateful features. I believe this is unnecessarily opinioned about how users are using groundhog. I propose we remove this requirement, and instead include an optional parameter to specify a grouping column. This means that a user with observations for only one entity will not need to make a dummy assetId column.

[jameslamb]

I agree with this completely.