[BUG] - _add_symbol_to_cell_input fails if the first row of cell definition doesn't contain at least cell number, material and density (if present)

[AlbertoBittes]

Describe the bug
_add_symbol_to_cell_input assumes that the first line of cell definition includes the material and the density(if cell is not void) of the cell:

        if parentheses:
            if new_cell.get_m() == 0:
                first_row[1] = first_row[1] + " ("
            else:
                first_row[2] = first_row[2] + " ("

In fact, the method assumes that first_row is a list with 2 or 3 elements depending on void/material cell.
In principle one could define only the cell number in the first row and go newline to define everything else. This is very unlikely however.

[dodu94]

good catch, do you plan to solve it in the PR you are preparing?

[AlbertoBittes]

I don't think I will have time for that, in the meanwhile I fixed that for our JT60 input