Initialize the spin matrix <tt>S</tt>.

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Usage: <tt> input = <i>style</i> initspin</tt>

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If <tt><i>style</i> = -1</tt>, then the entire matrix <tt>S</tt> is set to <tt>-1</tt>.
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If <tt><i>style</i> = 1</tt>, then the entire matrix <tt>S</tt> is set to <tt> 1</tt>.
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If <tt><i>style</i> = 0</tt>, then each entry in <tt>S</tt> is set to either
<tt> 1</tt> or <tt>-1</tt> at random.
