Check if variable name is correctly written in files by fxfilter.

[bascrezee]

While working on #4 it was found by @bascrezee that (for c2e) tqc = ds2.unknown.values had to be replaced by tqc = ds2.TQC.values. Current implementation supports both, but it should be checked why for @swester (for c1e) the latter option does not work. This could potentially be related to the external dependency fxfilter.