Work around Eigen's lack of proper move constr.

Eigen 3.3 (which isn't released yet) adds explicit move constructors;
for Eigen 3.2 and earlier, C++11's implicit move constructors break
Eigen, and so matrices/vectors need to be copied, not moved.  This
change provides a fairly useless move constructor for Eigen <3.3 which
just calls the default copy constructor; for Eigen >=3.3, this should
allow efficient move construction.