combine volcat files which are at the same time but have different ID numbers

[amcz]

volcat algorith may split up a plume into different pieces, each of which is in their own file.
In general, we want to be able to combine these both for data insertion, inversion algorithm, and evaluation.
They have the same valid time but different feature ids.
The feature id's do not track across time - for instance the same feature id does not necessarily correspond to the same piece of ash.