Change static const to static constexpr wherever possible

Since we require C++11, most of these consts can be constexpr.

Two `static const ld` remain non-compile-time-evaluable because
they depend on the runtime `log` function. One `static const cld`
remains non-compile-time because `std::complex<T>` doesn't become
constexpr until C++14.

models.cpp

@@ -20,7 +20,7 @@ EX namespace polygonal {
   #endif
 
   #if HDR
-  static const int MSI = 120;
+  static constexpr int MSI = 120;
   #endif
 
   typedef long double xld;