[HACK] Fix income group of population

[jamesturner246]

Feature request description

Hack the income group variable of each person to be a fixed configurable default value.

This is a HACK, and should not be merged into mainline.

Describe the solution you'd like

On initialisation, and later births, set income class to the configurable default.

Will need to read this default value from StaticLinear.json. so the schema files will also need updating.

Describe alternatives you've considered

N/A

Additional context

N/A