* This facilitates per-call selection of the underlying exchanger.

* This way the compiler is enabled to track safety of argument passing
  and further optimization potential unlocked.

* now the send and receive messages have to be freed by the caller

The new constructor xt_redist_repeat_asym_new generalizes
xt_redist_repeat_new by having different displacement
vectors for source and destination side.

So far xt_redist_repeat_new only allowed non-overlapping repetition of the input redist
although MPI allows more flexibility. This commit lifts this resitrcition. The user is now
responsible for a sensible type map.

* Do not iterate over full communicator but lists of ranks of
  component redists instead.

* This allows for simpler construction of the derived redists.

* Also exchangers now use the communicator passed to the constructor directly.
  This implies the caller has to make sure non-overlapping tag use.

* Also fix missing/malformed license headers and conditional inclusion macros
  where found.