Add target to run C++ demos

[garth-wells]

Add back a target via CMake to (i) build and (ii) run all C++ demos.

[chrisrichardson]

Some progress towards this in the branch chris/cmake-demos.
Remaining issues:

• OpenMPI requires the --allow-run-as-root flag. It would be preferable to remove this, and use an environment variable. This may now be possible in the latest OpenMPI: v4.0.x: Allow run-as-root if 2 envars are set open-mpi/ompi#5598

• Hyperelasticity demo fails to compile in complex mode: Either make it work, or prevent it from running.

[garth-wells]

See #274.

[garth-wells]

Fixes by PR #274.