v21.28

GILLES Sebastien

@sgilles

a4c3f189 · Issue tag v21.28. · Jul 15, 2021

- Update to MoReFEM v21.27.
- Fix XCode build (requires this [MoReFEM ticket](https://gitlab.inria.fr/MoReFEM/CoreLibrary/MoReFEM/-/issues/1663) not yet in MoReFEM v21.27).