Use literal string concatenation in getDataPath

This avoids an issue on some platforms (e.g.  conda-build 3 on
osx_arm64) where std::string(really_long_literal_string)+'/' ends
up with interstitial null characters that stop strcpy() and friends
from consuming the full, concatenated path.

src/library/detail.cpp

@@ -4,7 +4,7 @@
 namespace nuflux{
   namespace detail{
     std::string getDataPath(std::string fname){
-      return std::string(DATA_DIR)+'/'+fname;
+      return (DATA_DIR "/") + fname;
     }
   }
 }