cellSource: Writing the volume of the cellSource (sum(V)) for each time is now optional

Previous behavior which may be useful for moving-mesh cases can be
selected using the optional entry:
    writeVolume  yes;

The initial volume is written in the log and data file header e.g.:

 #   Source : all
 #   Cells  : 3829
 #   Volume : 9.943164e-01

Also added
    sumMag        | sum of component magnitudes