Make trend calculation a generator to accelerate viz interaction

[brownsarahm]

This pattern would mean that the visualization side does something like:

for result in labeled_df.get_subgroup_trends_1lev():
    # do viz stuff 

mehtod to update

the get_results would have yeild instead of return and it would calculate one each pass

[brownsarahm]

https://realpython.com/introduction-to-python-generators/