MappedBuffer

That is a fabulous hack of @psychocoderHPC and @ax3l to provide a buffer with mapped memory for kernel calls and basic host-side `BORDER -> GUARD` communication (like in `FieldE/B`).

You can not take this as an actual "pull request", it is more a point to share the branch and to improve it.

If this MappedBuffer works right, one could test it e.g. in `FieldE` by replacing it's `GridBuffer` (Device+Host memory) with a GridBuffer using (slower) mapped memory.

At the point of sharing this, it seems to get stuck (deathlock) during the asyncCommunication.