Fix bug #314:

- remove most of the metaprogramming kung fu in MathFunctions.h (only keep functions that differs from the std)
- remove the overloads for array expression that were in the std namespace

unsupported/test/mpreal_support.cpp

@@ -5,7 +5,6 @@
 #include <sstream>
 
 using namespace mpfr;
-using namespace std;
 using namespace Eigen;
 
 void test_mpreal_support()