text: Title: [BUG/ENH] deconcatenate_columns should inform users how many columns need to be provided
  Body: Currently, an `AssertionError` is raised if the number of columns provided to `new_column_names` is not correct, and the error message is not very informative, stating only, "number of new column names not correct."
  
  An improved error message would indicate how many columns should be provided.
  
  An even better API change might be adding a `autonumber_basename` kwarg, which automatically numbers the column names. For example, if the deconcatenated column adds 4 new columns that ought to share the same base name `serv`, it would automatically name them `serv1, serv2, serv3, serv4` (one-indexing or zero-indexing can be a kwarg as well!)