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.

This was deprecated several releases ago

The semantics for default parameters are as follows. If a value function has a
parameter that takes a default value, then an exception is raised if the user
ever tries to assign a value to this parameter via 'params'. These semantics
are chosen to eliminate the possibility that "forgotten" default parameters are
not silently overwritten.

The semantics for default parameters are as follows. If a value
function has a parameter that takes a default value, then an
exception is raised if the user ever tries to assign a value
to this parameter via 'params'. These semantics are chosen
to eliminate the possibility that "forgotten" default parameters
are not silently overwritten.