[FEATURE] Allow Adjustments to Birth Rates

[GregorSchroeder]

Is your feature request related to a problem? Please describe.
The previous Cohort Component Model allowed users to adjust birth rates as the model moved from the launch year to the horizon year such that users could explore different fertility scenarios. This required functionality was not initially included in the Birth Rate Input Module.

Describe the solution you'd like
A simple solution would be an input file (csv/JSON/etc....) of horizon year birth rates by race, sex, and single year of age that the model uses to grow/decay fertility at a constant rate from post-launch year to horizon year. This would also require a check to ensure the input file follows a required structure and contains all necessary race, sex, single year of age categories.

Note a similar Issue exists so potentially one utility could be applied to both Mortality and Fertility