Use C++11 standard features for detecting presence of Inf and NaN

Rasmus Munk Larsenrequested to merge
rmlarsen1/eigen-new:isnan_cxx11 into master

We now demand c+14 support of the compiler, and can rely on std::numeric_limits<T>::{has_infinity,has_signaling_NaN,has_quiet_NaN}.

Edited by Rasmus Munk Larsen

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