replace DataFrame.append for pandas.concat for pandas 2 compatibility

[fwaibl]

Description

I noticed that the tests are currently failing due to an incompatibility with pandas version 2. The solution is to use pd.concat instead of DataFrame.append.

Todos

Similar problems might arise in other files, but it is hard to find them reliably. I found the object NOE_violations in files/otherfiles/noe_output.py