Move the set_package_properties(PURPOSE) for the Alberta package to AddAlbertaFlags file

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 AlbertaGrid. 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.