As Boost is used in more and more modules and we recently would also profit from having Boost functionalities available in the Core (see T19492), we decided to finally add Boost as hard dependency.

We need to remove the MITK_USE_Boost option, adjust all build script files which mention Boost and add it as public package dependency to the Core instead. We will keep the possibility of using a system version of Boost and to specify binary Boost libraries.