Merge branch 'karlrupp/fix-and-improve-ilu'

karlrupp/fix-and-improve-ilu:
 Resolves a dangling const-ref issue with preconditioner tags.
 Fixes a bug in ILU0 (values in U not computed correctly).
 Improves performance for ILUT by about three-fold.
 Removes unnecessary temporaries for Block-ILU.