add optional track output scheme

[ManuelHu]

fixes #178

[ManuelHu]

example output:

evtid  trackid  parent_trackid  procid   particle  ...          zloc          px          py          pz     ekin
     0        1               0      -1 1000902280  ...  0.000000e+00   -0.000000    0.000000    0.000000 0.000000
     0        3               1       0 1000020040  ...  0.000000e+00  132.287874 -103.651817  110.515499 5.423120
     0        2               1       0 1000882240  ...  0.000000e+00 -132.287874  103.651817 -110.515499 0.096960
     0        5               2       0 1000020040  ... -4.594312e-09  174.504082 -103.121849   36.461075 5.685346
     0        4               2       0 1000862200  ... -4.594312e-09 -174.504082  103.121849  -36.461075 0.103504
   ...      ...             ...     ...        ...  ...           ...         ...         ...         ...      ...
     9       75              72       2         11  ... -1.152096e-01   -0.033582    0.063979   -1.016347 0.628870
     9       74              72       2         11  ... -9.015364e-02   -0.011724   -0.006307    0.006917 0.000220
     9       73              72       2         11  ... -9.015364e-02    1.284836    1.433781   -0.443888 1.529758
     9       88              73       4         11  ... -9.232236e-02   -0.088711   -0.289352    0.211443 0.119415
     9       71              42       0 1000822080  ... -1.577275e-08   -1.693407   -1.387681    1.429109 0.000018

with attrs['time']={'units': 'ns'}
with attrs['xloc']={'units': 'm'}
with attrs['yloc']={'units': 'm'}
with attrs['zloc']={'units': 'm'}
with attrs['px']={'units': 'MeV'}
with attrs['py']={'units': 'MeV'}
with attrs['pz']={'units': 'MeV'}
with attrs['ekin']={'units': 'MeV'}
 procid               name
      0 b'Radioactivation'
      2           b'compt'
      3           b'eBrem'
      4           b'eIoni'
      1            b'phot'

important note: to read the output as LH5, you will need legend-exp/legend-pydataobj#124