Added various other variants of MILU.

These versions are inspired by the ones used in SuperLU and the enums
to choose them have simuilar names, but without leading S (MILU_1-MILU_3).

The following variants are supported (chosen by the enum MILU_VARIANT):
ILU: plain ILU
MILU_1:  lump diagonal with dropped row entries.
MILU_2:  lump diagonal with the sum of the absolute values of the dropped row
         entries.
MILU_3: if diagonal is positive add sum of dropped row entrires. Otherwise substract them.
MILU_4: if diagonal is positive add sum of dropped row entrires. Otherwise do nothing

opm/autodiff/ParallelOverlappingILU0.hpp

@@ -396,6 +396,8 @@ class ParallelOverlappingILU0
 
 
 public:
+    enum{
+        };
     //! \brief The matrix type the preconditioner is for.
     typedef typename std::remove_const<Matrix>::type matrix_type;
     //! \brief The domain type of the preconditioner.