Convert this block (or columns of block) into a NumPy ndarray.
- Parameters:
columns (str | List[str] | None) – Name of columns to convert, or None if converting all columns.
- Returns:
A NumPy ndarray when a single column is selected, or a dict mapping
column names to ndarrays when multiple columns are selected.
- Return type:
ndarray | Dict[str, ndarray]