Fixed Gershgorin estimates.

CJ's version had a bug in it. It didn't properly bound the eigenspectrum. For
instance, using Validate/SinglePoint/Restricted/H2.inp with SCFMethod=(TC2),
it incorrectly bounds the spectrum to:

spectral bounds Gershgorin: [ -4.679890e-01, 6.655809e-01 ]
spectral bounds eigensolve: [ -4.911952e-01, 9.357483e-01 ]

The rewritten version now calculates the following bounds:

spectral bounds Gershgorin: [ -7.129550e-01, 1.231346e+00 ]

which agrees with what matlab thinks the bounds should be.