Generic halo exchange functions

[kaipartmann]

The current implementation of the halo functions is too limited. For the implementation of thermal models or surface correction, functions like

halo_exchange!(dh, (:position, :somefield))

would be very helpful.

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• [x] Renaming read_halo -> loc_to_halo (local values exchanged to halo fields; short: lth)
• [x] Renaming write_halo -> halo_to_loc (halo field values exchanged with += to local fields; short: htl)
• [x] Remove the check if it has lth or htl exchanges; always get these infos!
• [x] [MPIDataHandler] Generic function exchange_loc_to_halo!(data_handler, fields) with also the method exchange_loc_to_halo!(data_handler) which gets the fields from the loc_to_halo_fields(storage) function
• [x] [ThreadsDataHandler] Generic function exchange_loc_to_halo!(data_handler, fields) with also the method exchange_loc_to_halo!(data_handler) which gets the fields from the loc_to_halo_fields(storage) function
• [x] [MPIDataHandler] Generic function exchange_halo_to_loc!(data_handler, fields) with also the method exchange_halo_to_loc!(data_handler) which gets the fields from the halo_to_loc_fields(storage) function
• [x] [ThreadsDataHandler] Generic function exchange_halo_to_loc!(data_handler, fields) with also the method exchange_halo_to_loc!(data_handler) which gets the fields from the halo_to_loc_fields(storage) function
• [x] Add a generic buffer which gets buffers if a field is a halo field (maybe checked with is_halo_field)

[kaipartmann]

This is needed for #74, so it is a high priority now.