Planned - short term:
- fix -d option to allow unrestricted use
- fix -s option to allow unrestricted use ?
- inputs from filedescriptors
- print_tree does not work correctly when executed on non root node
- allow variables use in filename for file_input command

Planned - long term:
- good algorithm for adaptive window

Given up (see construct tests for some of these cases):
- efficient transfer of taktuk engine - actually not efficient (using the way I
  had in mind)
- sendv/recvv of different sizes - given up, not meaningful to TakTuk
- reduction of the number of concatenations - no easy gain still under
  investigation
