zlib compression of string variables

[wpreimes]

Currently pynetcf activates compression either for all or for none of the variables in a file. However, soon there will be an error when writing to disk with activated compression when a string variable is in a dataset. (when using the conda-forge version of netcdf4 it already is the case conda-forge/netcdf4-feedstock#136 (comment)).

We should therefore ensure that pynetcf sets zlib=False for string variables. I have recently added a test, that will fail in future https://github.com/TUW-GEO/pynetcf/blob/master/tests/test_base.py#L70