Merge branch 'karlrupp/feature-chow-patel-ilu'

karlrupp/feature-chow-patel-ilu:
 Implements a parallel incomplete LU preconditioner.
 Implements a parallel incomplete Cholesky factorization preconditioner.
 Both preconditioners use the same sparsity pattern as A.
 Proposed by Chow and Patel (Algorithms 2 and 3) in SIAM J. Sci. Comp.