adding a FromPtr/Load function to VecCore
Add missing backend function to VecCore.
In the spirit of adiabatic changes, I added only one function (FromPtr). This improved version is based on the discussion of my previous code (branch swenzel/veccore-loadstore). The function is now standalone and uses the available VecCore typetraits. Simple unit tests were added into the google-test backend test but they have to be extended and/or generalized (for which I need some assistance).