Support for SigWx in IWXXM format

As announced by UK WAFC in November 2023, the World Area Forecast System (WAFS) SIGWX charts are being modernised, and they will be disseminated in IWXXM format. UK WAFC prepared a test data set. Visual Weather and Aero Weather are now capable of visualising the IWXXM test data in a way that is consistent with the current BUFR-based SIGWX. We will continue improving the available options for this format, such as options for level synchronisation, as more test data becomes available.