Add an option for switching between storing block inverse or block factors storage in block Jacobi and Gauss-Seidel
Created by: aprokop
Ifpack2 now stores and applies the inverse of the matrix's block diagonal explicitly (#96 (closed)). Theoretically, this is a tradeoff. If block size is large enough, you can get better performance out of linear solves rather than full inversion when doing a limited number of iterations. So a user should be able to decide the appropriate approach for it.