Update set_package_properties for ARPack and SuperLU

In CMake, each package can define package properties like a DESCRIPTION what this package represents and a URL where to find the package. These are package properties. Additionally, CMake allows to set usage properties for a package, i.e, its PURPOSE. This should be set in each project (maybe only once) and not for each package, since its purpose might be different between projects.

This MR moves the definition of the PURPOSE property of ARPack and SuperLU. The effect is, that in the FeatureSummary at the end of a CMake run, it shows up only once, even if the package is found multiple times.

added buildsystem label

The original ARPack++ seems to be outdated. I have found out about this by searching for a URL to set for the ARPack packages. The new source https://github.com/m-reuter/arpackpp seems to provide a cmake package. Maybe this could be a replacement for our own CMake FindARPACKPP module.

Actually, what is ARPack used for in dune-istl? What is its purpose in the dune-istl module?

The purpose of the PURPOSE property is to give users of dune-istl an idea whether they need to provide this package or not.

merged

mentioned in commit 6543e8ce