[firedrake] C string expressions are gone

Dear all, As we pre-announced in December, C string expressions have just been deleted from Firedrake master. This means that the next time you update firedrake, they will stop working. If any users have not yet transitioned to UFL expressions, they will need to following the examples at: https://www.firedrakeproject.org/interpolation.html#c-string-expressions Anyone encountering difficulties should ask for help via the usual channels (GitHub, Slack, or this mailing list). Regards, David