Fix covid19datahub bug

[joseph-palmer]

Error caused by select(where( ~ !all(is.na(.x))). This was used to remove level columns not requested (e.g. if level 2 was requested this was designed to not select level 3 columns) by rejecting columns that are all NAs. The problem is that with the snapshot other columns that are used were composed of all NAs. This meant the code worked in full but not with the snapshot.

This PR replaces the removal of NA columns in cleaning with a removal of level columns not requested.

closes #347

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