Update some syntax for utility

[jrycw]

• For remove_pd_object, we can combine if and elif, since both of them will do the same thing, ret[n] = v.
• For enumerate_variables:
  • if len(variables) == 0 can be replaced by if not variables.
  • since constant_name and constant_vars will not be modified later, so we can construct a dictionary constant_d in advance instead of constructing the same dictionary in the following list comprehension.
• For enumerate_signal, if len(tuples) != 0 can be replaced by if tuples .
• For stop_early, we can construct stop_vars_set = {'sl_stop', 'ts_stop', 'tp_stop'} once, and use it for the following loop.

[koreal6803]

It is a great improvement! Thanks a lot!