Add rules to compile only 2d or 3d examples.

`make examples2d_deps` will compile the dependencies for the 2d examples.
`make examples3d_deps` will compile the dependencies for the 3d examples.
`make examples2d` will compile the 2d examples.
`make examples3d` will compile the 3d examples.
`make examples_deps` will compile the dependencies for every example.
`make examples` will compile every example.