Publish precipitation flux to wxml

[JanKianicka]

Dear Mike, as we were discussing already moths ago, we have made following custom modifications to titan in order to publish and use precipitation flux in nowcast parameters. If would consider it worthy for merging, please do so - this allows us to keep align with your development without need to merge customizations on our own. Thank you very much Jan Kianicka - Microstep-MIS, Bratislava, Slovakia

[mike-dixon]

Jan Apologies on taking so long to review this - I have not been properly checking the pull requests. I will merge this in - the change looks good. Thanks Mike