Parallel execution of Chi2 resampling

ResamplingChi2 accepts as last parameter the level of parallelism to use:
- 0 => do not use threaded code
- >0 => use this number of threads (ie 1 is not really usefull
   unless you want to test the threaded code without real parallelism)

Note that this parameter is *OVERWRITTEN* by the
ALTREE_PARALLEL environment variable as soon as it is defined.

The random generator has been changed: the one from the system
was a real bottleneck.

git-svn-id: svn+ssh://imag/users/huron/danjean/svnroot/claire/altree/trunk@467 cf695345-040a-0410-a956-b889e835fe2e