Error when using hierarchical columns with outer values ungrouped

[ForeverWintr]

A dataframe with hierarchical columns in where the outer labels are not grouped causes the an error in to_xlsx_worksheet. I think it's most easily illustrated with an example:

from table_compositor.table_compositor import build_presentation_model
from table_compositor import xlsx_writer
import static_frame as sf

cols = sf.IndexHierarchy.from_labels([(1, 1), (2, 2), (1, 2)])
frame = sf.Frame.from_element(1, columns=cols, index=[1, 2, 3])
df = frame.to_pandas()

# df:
# __index0__  1  2  1
# __index1__  1  2  2
# 1           1  1  1
# 2           1  1  1
# 3           1  1  1

pm = build_presentation_model(df=df)

# I'm setting worksheet to None because the error occurs regardless of whether it's provided.
xlsx_writer.OpenPyxlCompositor.to_xlsx_worksheet(layout=[pm], worksheet=None)

This produces the following traceback:

  File "/home/rutherford/.env38/lib/python3.8/site-packages/table_compositor/xlsx_writer.py", line 128, in to_xlsx_worksheet
    row_col_dict = cls._build_row_col_dict(
  File "/home/rutherford/.env38/lib/python3.8/site-packages/table_compositor/xlsx_writer.py", line 27, in _build_row_col_dict
    row_col_dict = GridLayoutManager.get_row_col_dict(
  File "/home/rutherford/.env38/lib/python3.8/site-packages/table_compositor/grid.py", line 135, in get_row_col_dict
    return GridLayoutManager.foldl(shifted_grid, f, dict())
  File "/home/rutherford/.env38/lib/python3.8/site-packages/table_compositor/grid.py", line 53, in foldl
    accum = GridLayoutManager.foldl(child, f, accum)
  File "/home/rutherford/.env38/lib/python3.8/site-packages/table_compositor/grid.py", line 45, in foldl
    accum = f(accum, cell.children)
  File "/home/rutherford/.env38/lib/python3.8/site-packages/table_compositor/grid.py", line 134, in <lambda>
    f = lambda accum, pm: {**accum, **to_row_col_dict(pm)}
  File "/home/rutherford/.env38/lib/python3.8/site-packages/table_compositor/presentation_model.py", line 147, in to_row_col_dict
    data = {
  File "/home/rutherford/.env38/lib/python3.8/site-packages/table_compositor/presentation_model.py", line 149, in <dictcomp>
    for offsets, value, style in data
builtins.ValueError: too many values to unpack (expected 3)