bandwidth.m: Add new function. * NEWS: Announce new function. * scripts/linear-algebra/module.mk: New function added to the build system. * scripts/help/__unimplemented__.m: New function removed from unimplemented. * scripts/linear-algebra/isbanded.m: New function added as documentation reference. * scripts/linear-algebra/isdiag.m: New function added as documentation reference. * scripts/linear-algebra/istril.m: New function added as documentation reference. * scripts/linear-algebra/istriu.m: New function added as documentation reference. * scripts/linear-algebra/bandwidth.m: New function. * linalg.txi: Add bandwidth function to manual.