Fix problem with detecting data type

We are currently skipping waveform files by inspecting the name of the file. However, for some Centaur and Pegasus data sets whose station name match a pattern used to filter waveform files, all their files are skipped. We fix this problem by checking for waveform channels instead of waveform files.

A unit test was skipped in this merge request. That test is not fixed because the problem is caused by #160 (closed), a fix for which would be out of scope of this merge request.

Closes #161 (closed).