Made TensorView copy constructor non-explicit to allow returning TensorView from a function.
Created diag(T) function that makes a TensorView of the diagonal of T.
Merge remote-tracking branch 'BTAS-emstoudenmire/rslice'
Revert "Merge remote-tracking branch 'BTAS-emstoudenmire/rslice'"

This reverts commit 2f46eb317a31a48717af07cb227de805413f44b4, reversing
changes made to b765bb5e58099eb15a35e5d71098b0f3eeb0bba4.