Netcdf4 structure

This PR is for creating a netCDF4 file filled with values from the PLEXPART output.  Although it is still not complete (lacking mainly attributes, so not very important), it already fills all the concentrations as well as wet and dry depositions (optionals).

 The netCDF4 file can be created either programatically (`reflexible.create_ncfile`) or by the means of the `create_ncfile` script, which takes the next arguments:

```
$ create_ncfile 
usage: create_ncfile [-h] [-n] [-W] [-D] [-d DIROUT] [-o OUTFILE]
                     [-c COMMAND_PATH]
                     [fddir]

positional arguments:
  fddir                 The directory where the FLEXDATA output files are.

optional arguments:
  -h, --help            show this help message and exit
  -n, --nested          Use a nested output.
  -W, --wetdep          Write wet depositions in the netCDF4 file.
  -D, --drydep          Write dry depositions into the netCDF4 file.
  -d DIROUT, --dirout DIROUT
                        The dir where the netCDF4 file will be created. If not
                        specified, then the fddir/.. is used.
  -o OUTFILE, --outfile OUTFILE
                        The complete path for the output file. This overrides
                        the --dirout flag.
  -c COMMAND_PATH, --command-path COMMAND_PATH
                        The path for the associated COMMAND file. If not
                        specified, then the fddir/../options/COMMAND is used.
```

TODO: complete the attributes and start documenting the new capability.
Not specific to this commit, but we should probably start to use pandas in places (e.g. read_releases and read_trajectories would likely be good candidates for returning a dataframe.