Non-mechanistic model with shifted cases can create explosive exponential growth

[sbfnk]

Summary:
Non-mechanistic model with a shifted cases prior can create explosive exponential growth

Description:
The case study at #545 has revealed an edge case when using the non-mechanistic model with large maximum generation time (and thus large seeding time) and an outbreak with small number of cases towards the end. In that case the create_shifted_reported_cases can create long (as long as the seeding time) periods of exponential growth because the rolling average goes below 1 but is then set to 1 with zeroes (interpreted as increase).

Reproducible Steps:
See closed PR linked above.

EpiNow2 Version:
1.4.9000