perf: Investigate GIL handling and threaded calculation performance after removing customPQConfig #66

Follow-up to #55.

Removing customPQConfig eliminates a callback from the C++ evaluation path into Python. This simplifies future GIL handling, but no performance improvements have been investigated yet as requested by #5.

For the 3.15 release, we should investigate the remaining work in libdiffpy and the diffpy.srreal bindings. These include but not limited to

• Identify where evaluation holds the GIL and which remaining callbacks or Python object accesses require it.
• Determine where the GIL can safely be released around C++ numerical calculations.
• TBD