Slepc 3.5.2 (update)

Update SLEPc to the newest version.
Add optional dependency on Arpack.
@dpo i had two installations in different custom folders outside of `/usr/local/`. To my knowledge, you can not install/symlink both versions at the same time in `/usr/local`. Unfortunately, it is not just a matter of different `.dylib`. In particular, some definitions in headers change depending on the version, i.e. `PETSC_USE_COMPLEX` is defined. Definitions of `PETSC_SCALAR` are also different. So one has to switch between the two if both are to be used in Homebrew. To my understanding, `PETSC_ARCH` and `SLEPC_ARCH` do not help anyhow in this situation. 

As it seems there is no other/better way to achieve it currently within Homebrew, i would stick with the proposed solution. If it fails one day, then one could give it more thoughts.