add MuPix converter plugins

This patch adds converter plugins for the MuPix2, MuPix3 and MuPix4 prototypes and a small utility to check raw data files called `FileChecker.exe`.

If your `data` directory points to your raw data files, the utility can be called with a list of run number, e.g.

    ./FileChecker.exe {6045..6050}

and it will check for each run that all events are readable, can be synced and which type of subevents it contains. The resulting output looks like this:

    run     valid  num_events  contains                   errors                    
    ------  -----  ----------  -------------------------  --------------------------
      6045   true       13131  MUPIX4,NI,TLU                                        
      6046   true           1  MUPIX4,NI,TLU                                        
      6047   true       14674  MUPIX4,NI,TLU                                        
      6048   true        7776  MUPIX4,NI,TLU                                        
      6049  false           0                             no events in the file.    
      6050  false          -1                             read error. 

Best,
Moritz