support user-defined preconditioners in PETScKrylovSolver
support user-defined preconditioners in PETScKrylovSolver

- new setup_precond option

vlukes/sfepy,sfepy/sfepy,rc/sfepy,lokik/sfepy,BubuLK/sfepy,rc/sfepy,rc/sfepy,lokik/sfepy,BubuLK/sfepy,BubuLK/sfepy,sfepy/sfepy,vlukes/sfepy,sfepy/sfepy,lokik/sfepy,lokik/sfepy,vlukes/sfepy