Cookiecutter for sample_restraint

Since the gmxapi sample_restraint code is now a subproject of the GROMACS project, it is no longer trivial to fork a new MD plugin project.

A reasonable solution could be a skeletal Cookiecutter repository that draws the sample_restraint subtree from the GROMACS repository. We could also separate some of the CMake logic, in that case, to better handle bundling of pytest, googletest, and testing infrastructure set-up.