Simple interactive plotting mode.

This PR adds an option for "simple" interactive plotting with Iplotclaw.  I often find that I'd like to perform some post-processing and plotting that is difficult or impossible within visclaw's framework, but is easy to write with pure matplotlib and numpy.  But I would like to use visclaw's interactive plotting mode, too.

With this PR, I can define a simple plot function that operates on a Solution object, like:

    def plotfun(sol):
        import matplotlib.pyplot as plt
        x = sol.state.grid.x.centers
        plt.plot(x,sol.q[0,:])

and then use it in interactive mode via

    import plotfun
    from clawpack.visclaw import Iplotclaw
    ip = Iplotclaw.Iplotclaw(setplot=plotfun,outdir='./_output',simple=True)
    ip.plotloop()

I simply pass my plotting function in place of setplot and use the keyword argument simple=True.  Overloading setplot here may be a bit hacky, and I'm not sure that everything works, though I have tested the basic functionality.