1.4.2

• Improved documentation
• Partial rebuild of the AMG hierarchy using new matrix, and reusing
  prolongation and restriction operators.
• Allow to use null-space vectors with MPI aggregation
• Implemented ILUP(k). The non-zero pattern of A^k is used to determine
  the fill-in, which may be beneficial for non-symmetric systems
• Implementation of experimental deflated solver
• prm.solver.ns_search=true ignores trivial solution for zero RHS.
  Useful in search of the null-space vectors of the system
• Published Docker image with compiled examples
• Removed SIMPLE preconditioner, which may be replaced with SchurPC
• Minor fixes and improvements.