Add typemap for SourceTime and src_time

[ChristopherHogan]

This turned out to be complicated. I thought the tyepmap would be as simple as

$1 = PyObject_GetAttrString(pyobj, "swigobj");

but that returns a PyObject, so I had to manually do the conversion from PyObject to C++ object that SWIG normally does, plus still allow a SWIG proxy object. Users can now pass a subclass of meep.source.SourceTime, or meep.src_time.
@stevengj @HomerReid @oskooi

[ChristopherHogan]

I just thought of a much nicer way to do this.

[ChristopherHogan]

The other option is to do something like

%feature("pythonprepend") meep::fields::add_volume_source %{
    new_args = [arg.swigobj if isinstance(arg, mp.source.SourceTime) else arg for arg in args]
    args = new_args
%}

which is much simpler. The disadvantage is that we would have to do this manually for every function that takes a src_time as an argument (and for any other high-level Python object that stores a swigobj). I think I prefer the typemap, even though it's more complicated, because we can write it once and it will work for every case. Reopening.

[stevengj]

This returns a new reference, so you need to Py_DECREF(swig_obj) after the tmp_res = ... to avoid a memory leak.

However, swig_obj = $input does not create a new reference, so you will need a Py_INCREF(swig_obj) there to balance out the decref.

[ChristopherHogan]

Good catch. Fixed.