add caiman.backup_dataframe(), change index parsing

[kushalkolar]

caiman.backup_dataframe() creates a backup copy of the dataframe

Modifier @_index_parser so that it doesn't allow int indexing, in case the integer indexing has been reset. This will also allow using get_parent(), get_children() between dataframes since it now only uses the UUID. @_index_parser will also accept pd.Series of a single batch item and return the UUID.