Input- and output file handling in meta_tune_threshold_simple

The new simple threshold tuning has several bugs with regard to input and output file handling:

• If a maskfile is given as input, it has to have a TDAC mask node, otherwise the script will crash. It should be able to also take a maskfile with only a disable mask
• Output files of the threshold scans are saved in the correct output directory, but the meta scan files ignore the output_directory setting in the testbench.yaml