[trace] simpler tracing interface, just one way to declare categories

details:
- TRACE_category updated to the new way platform file is traced:
     user must call TRACE_category *after* platform parsing;
     after MSG_create_environment or SD_create_environment (SMPI is ok)
  => such requirement allows the trace mechanism to figure out
     where to put the new categories in the traced type hierarchy
- TRACE_create_category killed, allowed the user to create a hierarchy
  of categories, but such hierarchy was not reflected in the categorized
  resource utilization

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/simgrid/simgrid/trunk@9168 48e7efb5-ca39-0410-a469-dd3cf9ba447f