Move from Constants to R-space

              Can't comment on Line 102 in this file. `/home/firedrake/firedrake/src/firedrake/firedrake/constant.py:73: FutureWarning: Giving Constants a domain is no longer supported. Instead please create a Function in the Real space.`

_Originally posted by @ddundo in https://github.com/mesh-adaptation/opt_adapt/pull/29#discussion_r1877775168_
            