Adding a tick box next to selected return periods

Currently, to stop a return period (e.g. 1 in 30 year) from running in a simulation, you have to delete it entirely. This is a problem when you're testing different storm scenarios but want to keep the full list of return periods for later — you end up re-typing them back in at the end of the design.

Suggested fix: Add a tick box next to each return period. Ticked = included in the run, unticked = skipped. This lets you switch periods on and off without losing the setup, and speeds up iterative testing (e.g. checking flooding at 1 in 30 without running the whole set every time).