Create function that computes post-infusion symptom levels

[bc]

As a User I want to be able to see how I respond to treatment in the days after an infusion.

helper functions:

days_since_infusion = compute_days_since_last_infusion()
computes_average_symptom_expected(days_since_infusion, symptom="anxiety")

mean([x[2] for x in anxiety_list])