fix: toDot => to_dot

[werifu]

When I ran python step-by-step.py tmp.dl --with-dot I found there were some call errors.

Traceback (most recent call last):
  File "path/problog/step-by-step.py", line 126, in <module>
    main(**vars(args))
  File "path/problog/step-by-step.py", line 76, in main
    print(gp.toDot(), file=f)
          ^^^^^^^^
AttributeError: 'LogicFormula' object has no attribute 'toDot'. Did you mean: 'to_dot'?

Besides, saveSDDToDot doesn't exist.

Traceback (most recent call last):
  File "path/problog/step-by-step.py", line 126, in <module>
    main(**vars(args))
  File "path/problog/step-by-step.py", line 93, in main
    nnf.saveSDDToDot(dotprefix + "sdd.dot")
    ^^^^^^^^^^^^^^^^
AttributeError: 'SDD' object has no attribute 'saveSDDToDot'

[rmanhaeve]

Thank you for this fix!