Halo exchange functions with fields as parameter

[kaipartmann]

The halo exchange functions now accept arbitrary storage fields.

Naming convention changed:

• read_halo_exchange $\Rightarrow$ loc_to_halo_exchange
• write_halo_exchange $\Rightarrow$halo_to_loc_exchange

New functions for MPIDataHandler

exchange_loc_to_halo!(dh::MPIDataHandler)
exchange_loc_to_halo!(dh::MPIDataHandler, fields::NTuple{N,Symbol}) where {N}
exchange_loc_to_halo!(dh::MPIDataHandler, field::Symbol)
exchange_halo_to_loc!(dh::MPIDataHandler)
exchange_halo_to_loc!(dh::MPIDataHandler, fields::NTuple{N,Symbol}) where {N}
exchange_halo_to_loc!(dh::MPIDataHandler, field::Symbol)

New functions for ThreadsDataHandler

exchange_loc_to_halo!(dh::ThreadsDataHandler, chunk_id::Int)
exchange_loc_to_halo!(dh::ThreadsDataHandler, chunk_id::Int, fields::NTuple{N,Symbol}) where {N}
exchange_loc_to_halo!(dh::ThreadsDataHandler, chunk_id::Int, field::Symbol)
exchange_halo_to_loc!(dh::ThreadsDataHandler, chunk_id::Int)
exchange_halo_to_loc!(dh::ThreadsDataHandler, chunk_id::Int, fields::NTuple{N,Symbol}) where {N}
exchange_halo_to_loc!(dh::ThreadsDataHandler, chunk_id::Int, field::Symbol)

Fixes #73