Optionally keep discarded eigenvalues in Spectrum

[emstoudenmire]

In the context of a truncation, the Spectrum object returned by svd and eigen could keep more information about the discarded eigenvalues beyond just the truncation error. Either this could always be done (perhaps the cost and memory footprint is never too bad) or done optionally when a certain keyword argument is passed. Then one could call

discarded_eigs(spec) # get the eigenvalues that were discarded by the truncation
all_eigs(spec) # get the original spectrum in its entirety
@test abs(sum(discarded_eigs(spec)) - truncerror(spec))