Slepc 3.5.2 (update)

Update SLEPc to the newest version.
Add optional dependency on Arpack.
@jacknagel different formulas with optional keg-only is, of course, an option. But cumbersome ;-)

Note, that the `version` does not depend on **some** other packages, it only looks like it. It is actually indeed a **different version** of the software, because it is built depending on how PETSc is built.

See #1320. SLEPc is conceptually an extension to PETSc, which itself can be built with complex-valued or real-valued algebra. Moreover, it can also be 32bit or 64bit. All that is the same software/package, but different compilation options which the user choose based on the application. Moreover, if there are different formula names, e.g. `petsc_real` and `petsc_complex`, nothing will stop Homebrew from using both simultaneously. So the user could try installing both and will end up with the conflicting headers and libraries. Therefore, I think, conceptually, `version` is a correct way to treat advanced compilation options of packages. It is a pity that this might get broken at some point. 

In either case, i only propose a solution which works for me and for my needs. It is up to @dpo to merge it or not ;-)