Fix/check append rep

[jkuhl-uni]

Hi, this is the second part to #275. With this PR, I'm introducing more sophisticated checking of the structure of append-mode files. In particular, the length of the header for each config can have a different length. This is particularly useful when the first config (that this not written with append mode) has slightly different output. Now the user gets a warning about this and only the "data part" of each chunk is checked to be of equal length.

Also in #275, there was a small bug that I corrected while reimplementing the check for the file structure.