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.

complex2.cpp

@@ -44,7 +44,7 @@ EX namespace brownian {
     c->landparam += val;
     }
   
-  static const int FAT = (-100); // less than 0
+  static constexpr int FAT = (-100); // less than 0
 
   void recurse(cell *c, int fatten_from) {
     int dl = getDistLimit();