Make more things plottable

This extends the notion of what is plottable to include arrays and
simple message types, in addition to individual fields in messages

In particular, if given a message subfield, it will plot the immediate numeric children, but will not recursively plot message fields.

This is useful for simultaneously plotting the x,y and z components of a vector, or all of the numeric fields in a debug message such as PID controller parameters.
len(get_plot_fields(topic_name)) > 0 will always be true, since get_plot_fields always returns two values
i think you meant to write
return len(fields) > 0, message