For each type of device, a *convTest function determines if the
current through that device is converged within tolerance,
and sets CKTnoncon if the current is not yet converged.
ASCRconvTest() erroneously subtracted old current from old,
rather than old from new, when evaluating convergence.
Also, since at least 3f5, the calling function NIconvTest assumed
that the worker functions indicated non-convergence through their
return value, so was ignoring the reports of current nonconvergence.
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