add first_multiple helper function

Eigen/src/Core/util/Memory.h

diff --git a/Eigen/src/Core/util/Memory.h b/Eigen/src/Core/util/Memory.h
index 6914870..b03bc37 100644
--- a/Eigen/src/Core/util/Memory.h
+++ b/Eigen/src/Core/util/Memory.h
@@ -470,6 +470,13 @@
   }
 }
 
+/** \internal Returns the smallest integer multiple of \a base and greater or equal to \a size
+  */ 
+template<typename Index> 
+inline static Index first_multiple(Index size, Index base)
+{
+  return ((size+base-1)/base)*base;
+}
 
 // std::copy is much slower than memcpy, so let's introduce a smart_copy which
 // use memcpy on trivial types, i.e., on types that does not require an initialization ctor.