#1532 Petsc wrapper: Refactor slightly the possible viewers to add more safety and enable reloading a matrix from a file
Reloading a matrix from a file is foreseen in PETSc only for two formats: HDF5 and binary. The former is not currently used, I will try the later as it comes with guarantees concerning portability (see https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Sys/PetscBinaryRead.html)