Update column name from mean_value to value --- appears names have changed

[aflaxman]

vivarium_public_health/src/vivarium_public_health/population/add_new_birth_cohorts.py

Line 227 in d5f135c

columns = ['year_start', 'year_end', 'age_start', 'age_end', 'mean_value']

This currently doesn't work for me, but changing mean_value to value and then also changing line 226 to filter for only rows with parameter of mean_value does.