Fix for Firedrake removing _Facets.markers
https://github.com/firedrakeproject/firedrake/commit/9125a65c0cb5bb671c62c33f05a0d42b983e06ed, in which 'deprecate' appears to mean 'remove without prior warning or suggested recourse'.
https://github.com/firedrakeproject/firedrake/commit/9125a65c0cb5bb671c62c33f05a0d42b983e06ed, in which 'deprecate' appears to mean 'remove without prior warning or suggested recourse'.