ILinePlot: Add default group for zoom sync
ILinePlot: Add default group for zoom sync

You currently have to specify a 'group' for the 'zync_zoom' parameter
to work. However a likely use case is to plot multiple signals with a
single ILinePlot call, and need them to be zoom-synced. For
example, suppose we want to plot the frequency of all CPUs
in a trace, on separate axes with zoom synced:

    ILinePlot(ftrace, signals='cpu_frequency:frequency',
              pivot='cpu', drawstyle='steps-post',
              sync_zoom=True, group='my_group').view()

Here the use of 'my_group' seems unnecessary since it only
occurs in one place.

Therefore use a hash of the ILinePlotGen object by default. This will
make the above example work with 'group' omitted.

ARM-software/trappy,bjackman/trappy,ARM-software/trappy,bjackman/trappy,ARM-software/trappy,bjackman/trappy,bjackman/trappy,ARM-software/trappy