This is a bit better for the "standard" batch.py script, as it writes NetCDF files.
- new class uvc_composite_plotspec, will replace a list of uvc_plotspec objects for output.
This lets us assign title to a composite plot and provides a place to put:
- new method write_plot_data() of class uvc_composite_plotspec.  For a composite plot, writes
an xml file describing its pieces (simple plots), and calls their methods to write them out.
- The class uvc_plotspec remains for backwards compatibility, but otherwise has been replaced
by uvc_simple_plotspec, which defines a single simple plot for output purposes.
Print a composite plot as an xml file (plus NetCDF files for the simple plots it contains),
using a new write_plot_data method.  There is no more list of simple plots, no more loop over
these plots printing NetCDF files one-by-one.