Soft definition periods

Definition periods in PolicyEngine Core are fixed: each variable must have exactly one time period for which it can have distinct values. Policy in country models often involves mid-year parameter changes or other ways that more flexible time period management might be useful.

Core has some shortcuts for this (e.g. set_input_divide, options=[ADD, DIVIDE]) but a more elegant solution to reduce the development burden on country models would just be to simplify all this into more autonomous Simulation behaviour: e.g. if I said my employment income was £30,000 per year, calculate my income tax in March, we really shouldn't need any country-specific logic describing how the time periods interact with each other here. The only thing we need to specify at the country level is whether each variable is a stock or a flow, because this implies different behaviour is correct:

• wealth in 2022 is 450k => wealth in 2022-01 is 450k
• income in 2022 is 30k => income in 2022-01 is 2.5k

The issue proposes more autonomous Simulation behavior for flexible definition periods, including stock-versus-flow handling, and mentions set_input_divide and options=[ADD, DIVIDE]. Start by reading those existing mechanisms and the Simulation behavior around time periods. The desired design and completion criteria are not specified, so clarify the scope before implementing.