Set pmech and efd in Genrou hard_coded_residual

In the Genrou hard_coded_residual test, since we are not calling gen.initialize() and there are no control devices, pmech_set_ and efd_set_ are never initialized. They default to 0 in most situations I have tested, but this is not guaranteed. I've been experimenting with another way of handling those set points that led to an undefined behavior. Initializing those values to 0 where they are declared makes the test pass, but I doubt that's the behavior we want.

https://github.com/ORNL/GridKit/blob/bcca730dd74a99d5c1416fa1065199b751bed2fe/tests/UnitTests/PhasorDynamics/GenrouTests.hpp#L163

Also see the discussion in #266 for the case where signal nodes are used.

cc @joshua-x-7 @abirchfield @pelesh