only PETSc distributed vector from linsys

- linsys_BDDC and linsys_PETSC now only return distributed Vec type with
  solution
- removed get_whole_solution and set_whole_solution methods from linsys
- removed solver_indices for reordering of unknowns - it is fully
  substituted by par_to_all scatter
Fix creating of serial scatter in unsteady case.

- move the call into create_linear_system
Resurrect unsteady implicit test.
got rid of one copy

- by creating a PETSc vector with a local array
Merge branch '1.8.0_master' into JSI_bddc
Merge branch '1.8.0_master' of github.com:flow123d/flow123d into 1.8.0_master