review different age groups used

[tomjemmett]

there are at least 3 I know off the top of my head:

• the main groupings used by the model data
• in inputs we use 5 year age bands
• we use a different set of groupings in reference/population_by_imd_decile.py

do we need these different groupings, or could we consolidate?

[tomjemmett]

no longer using age bands in the inputs data