Do not redefine `n_dates` if defined in namelist

If `n_dates` has been defined in the namelist then it is used by
`read_column` and, after the call to `read_column`, it is already the size
of `ishape_last`. It is clearer not to redefine it.

Overlap/eddy_graph.f90

@@ -93,7 +93,7 @@ program eddy_graph
      copy = merge(dist_lim, 0, periodic)
      call read_column(ishape_last, &
           file = trim(shpc_dir) // "/ishape_last.txt", last = n_dates)
-     n_dates = size(ishape_last)
+     if (n_dates == huge(0)) n_dates = size(ishape_last)
      call assert(n_dates >= max_delta + 1, &
           "eddy_graph: n_dates should be >= max_delta + 1")
      call assert(n_proc <= n_dates / (max_delta + 1), &