introduce a new LvalueBit flag and split DenseCoeffBase into three level of accessors

Eigen/src/Core/util/Constants.h

@@ -139,6 +139,14 @@ const unsigned int DirectAccessBit = 0x20;
   * means the first coefficient packet is guaranteed to be aligned */
 const unsigned int AlignedBit = 0x40;
 
+/** \ingroup flags
+  *
+  * Means the expression is writable. Note that DirectAccessBit implies LvalueBit.
+  * Internaly, it is mainly used to enable the writable coeff accessors, and makes
+  * the read-only coeff accessors to return by const reference.
+  */
+const unsigned int LvalueBit = 0x80;
+
 const unsigned int NestByRefBit = 0x100;
 
 // list of flags that are inherited by default
@@ -234,6 +242,10 @@ enum {
   IsSparse
 };
 
+enum AccessorLevels {
+  ReadOnlyAccessors, WriteAccessors, DirectAccessors
+};
+
 enum DecompositionOptions {
   Pivoting            = 0x01, // LDLT,
   NoPivoting          = 0x02, // LDLT,