Check column number consistency in study and assay files

[mkuhring]

Came across study files with different number of fields per row. E.g. header is longer than data row. Maybe empty fields at the end of the row get cut of by some software or script? We should check for this and create a more useful error message for the user. Currently, it is just "list index out of range". Should we also correct this automatically. I.e. fill up shorter rows with empty fields (similar to #34). I would prefer an actual error, because we can't assume that just empty fields are missing and not actual data.