assign for AllAtOnceFunction and AllAtOnceCofunction should use _vs_op for all sources.

[JHopeCollins]

In #158 I tried to change the assign implementation to just use the _vs_op utility method to reuse the logic for dealing with assigning from another AllAtOnceFunction, a Vec, or a Function. This passes all assign tests.

However, test_allatoncesolver::test_solve_heat_equation fails if _vs_op is used to assign from a Vec. The only difference is whether the aaofunc.global_vec is accessed via read/write access or just write access. This shouldn't make a difference because the vec is overwritten with the new value anyway, but it makes the test fail.
For the time being I've just left the old assign implementation for Vec. Although using _vs_op is just a nice way of reusing code, we should track down why it fails because there may be some other underlying issue.