Replace Arraytypes with xtensor in Proxy interface

Replace all occurrences of idg::Array1D, idg::Array2D etc. from idg-util and idg-bin with equivalent xtensor code. The aocommon::xt::Span xtensor wrapper is used in all interfaces, such that a Proxy can take care of memory allocation (and is in charge of free'ing it upon Proxy destruction), while the other code can use the convenience functions from xtensor.

The intention was to do a direct port, without altering any functionality. This was not possible everywhere, though. For instance, in Plan, some code has been changed (improved) now that some data structures have changed. From the Proxy onwards, the ArrayType's are still used, e.g. in idg-api. This code will be refactored in a follow-up MR.