Fix the return type of Trilinos's VectorReference::operator=.
Fix the return type of Trilinos's VectorReference::operator=.

This was non-intuitive, though it did not make a difference given the particular
semantics of vector references. Fix it anyway.

EGP-CIG-REU/dealii,kalj/dealii,danshapero/dealii,sairajat/dealii,kalj/dealii,naliboff/dealii,naliboff/dealii,danshapero/dealii,naliboff/dealii,naliboff/dealii,EGP-CIG-REU/dealii,kalj/dealii,danshapero/dealii,sairajat/dealii,danshapero/dealii,sairajat/dealii,kalj/dealii,EGP-CIG-REU/dealii,EGP-CIG-REU/dealii,sairajat/dealii,kalj/dealii,naliboff/dealii,sairajat/dealii,danshapero/dealii,kalj/dealii,naliboff/dealii,danshapero/dealii,naliboff/dealii,sairajat/dealii,EGP-CIG-REU/dealii,danshapero/dealii,EGP-CIG-REU/dealii,kalj/dealii,sairajat/dealii,EGP-CIG-REU/dealii