Ability to run models for a subset of years

## Is your feature request related to a problem? Please describe

It's annoying that if a model is defined for a certain set of years, you HAVE to run the model for all those years (in most cases). If you tried removing years from `milestone_years` (say you only wanted to run the first year just for an initial test), it would complain about some of your inputs referring to milestone years that don't exist (e.g. an asset defined for 2040).

## Describe the solution you'd like

I think we should allow:
- running only the first n milestone years
- running with reduced milestone year granularity (e.g. if the model is defined with a 1-year granularity, an ability to run it with 10-year granularity)

Some easy way for users to specify this

## Describe alternatives you've considered

None

## Additional context

Hopefully there wouldn't be any big implications that would make this difficult. Needs some thought though.

Start by tracing how milestone_years and inputs referring to milestone years are validated, then follow the model execution configuration. Done means users can select only the first n milestone years or use a reduced milestone-year granularity without invalid-input errors.

Written by the indexing model from the issue text.