Slepc 3.5.2 (update)

Update SLEPc to the newest version.
Add optional dependency on Arpack.
@dpo yes, i do install SLEPc and PETSc twice. 

@jacknagel i supposed, currently, there is no other "clean" way to achieve the desired effect - two separate installations of formula with different build options? One could modify the default `prefix` to install in separate folders, but i think this approach is even worse. Not sure if `brew unlink` and `brew switch` will work in such a case.