early NaN detection

[nicolasaunai]

In MHD we want NaN detection:

• to be as close as possible as where they can be produced
• to be optionally disabled as they take runtime, TBC whether it's by a compile flag or python dictionary option. Probably not debug vs release because this is typically something that might happen after a while and in release.

[PhilipDeegan]

It's not working on clang, but this PR might solve this