Update `health-plots.R` according to the new data processing steps

[SaranjeetKaur]

@jzhu20 I have requested your review on this PR. There are quite a few changes that are taking place with this PR:

• The riskfactors_diff(), inc_cum() and burden_disease() functions have been updated according to the new steps in the data-process.R file.
• For above three functions, the users can now set the y-axis aesthetics like the limits, breaks, and labels.
• Please let me know if you want any changes in the following functions or if they should be completely removed: riskfactors(), inc_diff(), life_exp()?