Gracefully handle NaN/inf values in fast_atoreal_move

There are legitimate cases where inf/nan values are embeddded in a mesh.
Such values should not cause loading to fail, and indeed, previous versions of
Assimp supported their existence. Update the new fast_atoreal_move method
to gracefully parse such values, allowing case-insensitive checks for "NAN",
"INF" and "INFINITY" as per the atof guidelines found at
http://en.cppreference.com/w/cpp/string/byte/atof.

Note that the inf/nan text parsing is fairly loose, but is fast for the general
case and should handle most legitimate inf/nan values.