Add max_digits10 in NumTraits for mpreal types.

unsupported/Eigen/MPRealSupport

diff --git a/unsupported/Eigen/MPRealSupport b/unsupported/Eigen/MPRealSupport
index ba9d7c9..322ff45 100644
--- a/unsupported/Eigen/MPRealSupport
+++ b/unsupported/Eigen/MPRealSupport
@@ -95,6 +95,10 @@
     return std::numeric_limits<Real>::digits10(Precision);
   }
   static inline int digits10(const Real& x) { return std::numeric_limits<Real>::digits10(x); }
+ 
+  static inline int max_digits10(long Precision = mpfr::mpreal::get_default_prec()) {
+    return std::numeric_limits<Real>::max_digits10(Precision);
+  }
 
   static inline int digits() { return std::numeric_limits<Real>::digits(); }
   static inline int digits(const Real& x) { return std::numeric_limits<Real>::digits(x); }