stats: remove ns3::NaN symbol, use std::nan("") instead

This is prompted by a tiny piece of !2229 (closed).

src/stats/data-calculator.h declares

• ns3::NaN
• ns3::isNan()

These should be deprecated, and existing uses changed to std::nan("") and std::isnan(), respectively.

In addition src/stats/data-calculator.cc defines

• ns3::zero

to generate a NAN. Since this is visible only in the compilation unit it can just be removed.