untie: unlink previously linked parameters

Untie the specified parameter from any links to other parameters.

Syntax:      untie <param range>

where  <param range> is of the form

<param range> =::  [modelName:]<param #>

Parameters previously linked together with commands such as

XSPEC12> newpar <param spec>

are unlinked. The parameter will retain its current value for the next fit.