Move definition of `single_grid_uv` later

Move definition of `single_grid_uv` after call to
`read_grid_inst_eddies`. Preparing to move `stag_[uv]` to
`read_grid_inst_eddies`.

Inst_eddies/input_ssh.f90

@@ -63,7 +63,6 @@ contains
     periodic = .false. ! default value
     print *, "input_ssh: Enter input_ssh_nml:"
     read(unit = *, nml = input_ssh_nml)
-    single_grid_uv = all(stag_u == stag_v)
     call nf95_open(ssh_fname, nf95_nowrite, ncid)
     call read_grid_inst_eddies(periodic, stag_u, stag_v, ncid, leave_trace)
     if (periodic) call assert(4 * max_radius(1) < nlon, "input_ssh: max_radius")
@@ -71,6 +70,7 @@ contains
     allocate(ssh(1 - copy:nlon + copy, nlat), u(1 - copy:nlon + copy, nlat), &
          v(1 - copy:nlon + copy, nlat))
     corner_u = [1. - copy + stag_u(1), 1. + stag_u(2)]
+    single_grid_uv = all(stag_u == stag_v)
     if (.not. single_grid_uv) corner_v = [1. - copy + stag_v(1), 1. + stag_v(2)]
 
     ! Read ssh, u and v: