Add mdfwriter.py script

It can be used for converting data from ROOT formats to MDF but also to subset MDF data (e.g. first N events).

LHCb/run mdfwriter.py -h

usage: mdfwriter.py [-h] -o OUTPUT [-f] [--db-entry] [--start START] [-n NEVENTS | --stop STOP] files [files ...]

Convert data to MDF format

positional arguments:
  files                 Input filenames or TestFileDB entry

optional arguments:
  -h, --help            show this help message and exit
  -o OUTPUT, --output OUTPUT
                        Output filename
  -f, --force           Overwrite output file if it exists
  --db-entry            Interpret the file as a TestFileDB entry
  --start START         First event to write
  -n NEVENTS, --nevents NEVENTS
                        Number of events to write
  --stop STOP           Last event to write (exclusive)

This kind of generalizes ExampleXDigiToMDF.py, so we might decide to drop it.

/cc @sstahl @chasse