Confirm that SetGlobalStartEndYears() works as expected

[celiot-IDM]

Some code tidying, and some unit test confirmation that SetGlobalStartEndYears() works correctly.

[celiot-IDM]

Will fix #24

[celiot-IDM]

With this code we should probably update our standard examples to include a call to SetGlobalStartEndYears():

library(ehep)

ehep::InitializePopulation()
ehep::InitializeHealthcareTasks()
ehep::InitializeScenarios()
ehep::InitializeStochasticParameters()
ehep::InitializeSeasonality()

scenario <- "MergedModel"

ehep::SetGlobalStartEndYears(2025, 2055)

results <-
  ehep::RunExperiments(scenarioName = scenario,
                       trials = 100)

I've confirmed that the start/end years are carried through to nice plots like this (100 population growth trajectories, with 95% CI around the mean):

[MeWu-IDM]

We should update regression tests to use https://github.com/InstituteforDiseaseModeling/Ethiopia-HEP-Capacity-Analysis/blob/charles/issue24/tests/testthat/test_lean_scenario.R to use ehep::SetGlobalStartEndYears with default years, @celiot-IDM do you want me to update this or you can add it?

[celiot-IDM]

@MeWu-IDM : I'm happy for you to make that change! Thanks for calling it out.

[MeWu-IDM]

Updated the regression tests and it is passing with default values. This looks good to me