[iso2nf.py] Only shell elements

[kowalskiw]

Script does not work when there is no BEAM elements. The following error message is printed:
Traceback (most recent call last): File "D:\fireeng-tools\structures\iso2nf.py", line 661, in <module> run_user_mode(n, args) File "D:\fireeng-tools\structures\iso2nf.py", line 598, in run_user_mode m.make_thermals(arguments.config) File "D:\fireeng-tools\structures\iso2nf.py", line 419, in make_thermals tems, tshs, t_end = read_mech_input(self.input_file) File "D:\fireeng-tools\structures\iso2nf.py", line 74, in read_mech_input if len(tems[element[-1]]) < 3: KeyError: '1'

To be fixed and tested for structures with SHELL elements only.