Plotting Bessel function

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Thanks, this works well. By defining the lambda function outside the plot command, I can use it also in find_local_maximum and find_root. I will check the documentation to better understand why a lambda function is needed for the Bessel functions.

I think this is because the in the current implementation the Bessel functions define their own `plot` method and the options supported aren't the same as the top-level plot function. hopefully this can be improved sometime soon. It looks like it would be very easy to pass along a dictionary of options through the Bessel class's plot method: http://hg.sagemath.org/sage-main/file/d06cf4b2215d/sage/functions/special.py#l1176

This is now http://trac.sagemath.org/sage_trac/ticket/13553

Thanks for clarifying this. So using a lambda function in the plot call allows for the standard options to be passed to the plot method.

@benjaminfjones - as you are certainly aware, making Bessel functions symbolic would also fix this - e.g. http://trac.sagemath.org/sage_trac/ticket/4102 for Bessel J.