ENH: improve handling of string passed for columns parameter in e.g. copy_layer

[theroggy]

Now this is treated as any other iterable, requesting columns for each character in the string.

Easiest for the user would be that a single string is also supported and is treated that there is only one column to be retained, so everywhere before a columns - like columns is used put something like this:

if isinstance(columns, str):
    columns = [columns]

In practice this will have to end up on several places:

• _ogr_util.vector_translate
• fileops._append_to_nolock
• _geoops_sql._single_layer_vector_operation
• _geoops_sql._two_layer_vector_operation
• _geoops_gpd._apply_geooperation_to_layer
• _geoops_gpd.dissolve
• ...?