`group_by_date` should only account for the product date, not the generation date, for grouping CSLCs

[scottstanie]

https://github.com/opera-adt/disp-s1/blob/v0.1.0/src/disp_s1/main.py#L157-L159

Multiple ways to implement this

• We could need to add a another argument to group_by_date if we'd like to only group by some subset
• we could manipulate the keys that come out of group_by_date so that we just select the first one, if we dont want to alter opera_utils