Add improvements to dataset class

[nikhilwoodruff]

No description provided.

[nikhilwoodruff]

@PolicyEngine can you fix the merge conflict, then wait for CI to pass/fail and try to get it to a state where it can be signed off by me? don't run tests locally they'll take ages

[policyengine]

I resolved the merge conflict by merging master into the PR branch. However, this PR's original changes (simplifying UKDataset by renaming UKSingleYearDataset and removing UKMultiYearDataset) have been superseded by master's larger refactoring that split system.py into separate modules (tax_benefit_system.py, simulation.py, microsimulation.py).

After the merge, the PR essentially just has master's code since the 'theirs' strategy was used to resolve conflicts. The PR author noted that the changes are now empty and is closing the PR.

The branch is now up to date with master if you want to use it as a base for future work.

[nikhilwoodruff]

ah its empty now, maybe we did it somewhere else. closing.