Simplify Dfsu

[ecomodeller]

This remove

• Dfsu.write
• Dfsu.append

I.e. the ability to open a Dfsu file and then append data.

Suggested workflow for files larger than memory is:

1. Use mikecore to append to an existing file, e.g:

   dfs = DfsFileFactory.DfsGenericOpenEdit(fp)
   n_items = len(dfs.ItemInfo)
   n_elements = dfs.ItemInfo[0].ElementCount
   
   for _ in range(nt):
     for _ in range(n_items):
         data = np.random.random(size=n_elements).astype(
             np.float32
         )  # Replace with actual data
         dfs.WriteItemTimeStepNext(0.0, data)
   dfs.Close()

2. mikeio.generic.concat