ad: Split advection/diffusion example into smaller examples.

Signed-off-by: Matthew Emmett <memmett@gmail.com>
encap: Change get_q to get_state.

Signed-off-by: Matthew Emmett <memmett@gmail.com>
encap: Change set_q to set_state, get_pg to get_saved_state etc.

Signed-off-by: Matthew Emmett <memmett@gmail.com>
ad: Remove old advection_diffusion.cpp.

Signed-off-by: Matthew Emmett <memmett@gmail.com>