write_workbook errors with all integer table

[MarkPaulin]

Describe the bug
write_workbook throws an error if a table has only integer columns.

To Reproduce
Steps to reproduce the behavior:

import gptables as gpt
import pandas as pd
df = pd.DataFrame({"a": [0], "b": [1]})
table = gpt.GPTable(df, table_name="table", title="numbers")
gpt.write_workbook(filename="example.xlsx", sheets={"table 1": table})

Expected behavior
write_workbook should work with tables that have all integer columns.

Additional context
The error comes from this line, which is checking if any cells contain only special characters. The way it works currently assumes gptable.table.stack() is a series of objects, but that isn't the case if all the columns are numeric.

I can submit a PR to bypass this check whenever gptable.table.stack() is a numeric type, if you think that's a suitable solution.

[rowanhemsi]

Thank you for flagging this Mark and sorry it has take so long for us to get back to you. We will take a look and try to get this fixed as soon as possible