Introduce typed tasks in SimDag

Introduce typed tasks in SimDag
- Specify its kind and cost at creation.
- At scheduling, just give where it should be placed, and the cost for
  each involved resource is automatically computed.
- Existing constructors so far (more to come of course):
  - SD_task_create_comm_e2e() for end-to-end communication
  - SD_task_create_comp_seq() for sequential computation
- Use SD_task_schedulev() / SD_task_schedulel() to schedule them.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/simgrid/simgrid/trunk@6725 48e7efb5-ca39-0410-a469-dd3cf9ba447f