Modify a UM restart file to work with new vegetation distribution

@tammasloughran - thanks for the helpful list.
I think all the 3xxx fields can be ignored. I’m pretty sure the model will start OK without them.
Snow fields are probably OK all set to zero, but it looks like soil temperature should then be set to 273.16 where that pft exists - so perhaps it is easiest just to average as with other fields.
Frozen soil fraction is probably also best averaged.