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.
shouldn't this simply be
    from rqt_py_common import topic_helpers