Use repr instead of str for printing column and index

[richardlin047]

The history cell will output code that might error if it uses columns with number names because of how we format the variable names.

The sort column code snippet is currently:

df.sort_values('{self._sort_field}', ascending={self._sort_ascending}, inplace=True)

This assumes the column name is always a string. To allow columns with numbers as the name, it should be:

df.sort_values({repr(self._sort_field)}, ascending={self._sort_ascending}, inplace=True)

This should be fixed for wherever there is a use of '{variable}' for columns or rows.