[ENHANCEMENT] Add a new risk factor - Ethnicity

[jzhu20]

Feature request description

Ethnicity is added as a risk factor.

Describe the solution you'd like

Prevalence of ethnicity should be given in configuration.
Individual ethnicity remains unchanged over life course. Newborns' ethnicity status is allocated with respect to ethnicity prevalence.
Ethnicity will affect individual income class.

Describe alternatives you've considered

NA

Additional context

NA

[jamesturner246]

Some things to consider:

• For the best model generality, we should enable the code to account for 'all' ethnicities, but we need to agree on a conventional, widely recognised/used, list of ethnicities. We should aim to keep this list stable, so consider what granularity future models might need.