[FEATURE] Allow Adjustments to Death Rates

[GregorSchroeder]

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

Describe the solution you'd like
A simple solution would be an input file (csv/JSON/etc....) of horizon year death rates by race, sex, and single year of age that the model uses to grow/decay mortality at a constant rate 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