Boost timer requires chrono

[Prof Garth Wells]

Boost timer is in the DOLFIN public interface, as is marked as such in CMake files. But, it seems to require the Boost chrono library and CMake is not identifying this dependency. Need to add chrono to the list of libraries to avoid library issue failure.

[Martin Sandve Alnæs]

Maybe better to switch to std::chrono: http://en.cppreference.com/w/cpp/chrono

[Prof Garth Wells]

It's used internally by boost::timer, not by us directly.