[Python wheel] Currently, NML support is disabled.

[brenthuisman]

The binary wheels for v0.5.2 were built without NeuroML support. The precise reason is still unclear, because for this reason NML support was set to enabled by default, which I locally verified. It explains why the missing libxml2 error started showing up in #1784.

Solution:

• Change build to actually enable NML support.
• Add testing to the wheel workflow (e.g. run scripts/run_python_examples.sh).
• Modify config() to show whether or not nml was compiled in.
• Add the script and documentation for local testing of the manylinux wheel builds to the repo.