Added tests for nc4 and nc4z input
Start to fix issues with nc4 and ncl 6.1.1 (using 6.1.2)

background is the variable names retrieved by getfilevarnames. NCL 6.1.1
introduced a leading '/' for nc4 files, to cover the variable-groups
feature of nc4. I'd like to use nc4 with zip compression to save IO time
in post-processing ICON output, but currently the script does not work
properly when reading variables from the file handle.

http://www.ncl.ucar.edu/Document/Functions/Built-in/getfilevarnames.shtml

Possible fixed could be

* insert a leading '/' when using nc4 (we do not use variable groups, but the root group os instill indicated by this, allthough ncdump does not show any '/')
* use this technique of selecting a group: http://www.ncl.ucar.edu/Document/Manuals/Scripts/nc4_group.ncl encapsulated into an addfile wrapper
Add script for smaller tests of seprate ncl functions