GStreamer 1.0: Fix compiler warnings

Fixes new compiler warnings in the gstreamer 1.0 upgrade

I've split out each warning fix into a separate commit:

• Fix unsigned/signed comparisons, I took the opportunity to refactor a few iteration variables to be local to the loop.
• GCC complains about 'missing braces', which seems to refer to structs which expect an array (or something similar) being initialized with scalar values. This occurs when using PyObject_HEAD_INIT(NULL), which as best I can tell should be the same thing. See https://docs.python.org/3/c-api/structures.html#c.PyObject_HEAD_INIT
• Fixes an implicit cast. A float * gets cast to COMPLEX_F *. Essentially an array of pairs of floats is flattened in a data structure that only seems to support fundamental types. So I cast it directly. Maybe a 2D array would be more appropriate??
• GstURIType changed.
  It used to be https://www.freedesktop.org/software/gstreamer-sdk/data/docs/latest/gstreamer-0.10/gstreamer-GstUriHandler.html#GstURIHandlerInterface
  Now it's https://www.manpagez.com/html/gstreamer-1.0/gstreamer-1.0-1.10.0/gstreamer-GstUriHandler.php#GstURIHandlerInterface
• Here I Maybe went too far:
  A function forward_src_event didn't necessarily initialize success. This is gstreamer_python_upgrade code which was somewhat copied from another location to replace needlessly complicated and hard to upgrade code in spiir-O4-EW-development. However, on fixing this warning, it was clearly a bit broken, so I've reworked it in a way that I think is reliable.

There's only warnings left in bank generation, but that entire directory will be removed in the next MR.

I won't try to build in CIT at this stage...

Tests

/fred/oz996/tdavies/spiir_project/sources/testing/gout/py3/MR97_tests