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  `integrate` is a more accurate name for a function that integrates
   the spectral density. Docstrings and tutorial updated.

If "expr" is already a SymPy object and "locals" are provided the
RuntimeWarning will be triggered.

closes #125

The default relwidth was 0.03.  This means 33 "widths" per longest
plot dimension, and 67 stream lines (we show 2 streamlines per
width by default).

67 streamlines are too much for small square plots.  Increasing
relwidth to 0.05 produces nicer looking results and the current
plotter also runs much faster!

The purpose of keyword-only args is to allow functions that take a
variable number of args but also a variable number of "options" without
having to resort to (*args, **kwargs).  Builder value functions do not
fall into this category.  Hence, there is no reason to pay the price (in
terms of flexibility and efficiency) for not much gain.

The site() method was never widely advertised, so no one will miss it.