This adds a get<N>() function which returns the n'th element
of an aggregate type (e.g. vector type, pair, tuple).
This unifies the functionality of, and replaces, the get_pair()
and vector_component() functions.
refs kylelutz/compute#9
device, context, and queue are initialized statically in `context_setup.hpp`.
With this change all tests are able to complete when an NVIDIA GPU is in
exclusive compute mode.
Side effect of the change:
Time for all tests to complete reduced from 15.71 to 13.03 sec Tesla C2075.