Let Netcdf Concatenater add time steps without rewriting previous time steps.

[erikpelgrim]

Currently the Netcdf Concatenator reads and writes the whole variables every time it is run.
In case the time variable already has an unlimited dimension it would be more efficient if the time steps are added to the end without having to read and rewrite all time steps before that remain unchanged.

First start with an investigation wheter or not this is achievable within a acceptable amount of time and effort.