Reworked PE/threads relation (Closes #725):

· There is no threads per PE concept anymore.
· A PE can have 2 threads and another one just one thread.
· SMP threads are binded to their PE's CPU id.
· Added a new parameter, NX_THREADS. OMP_NUM_THREADS is an alias.
· NX_PES only specifies the number of PEs. If you want to run with 4 threads on just 2 PEs, use NX_THREADS=4 and NX_PES=2.
· If NX_THREADS is not specified, it will be set to the number of PEs.
· If NX_PES is not specified, it will use the same number as threads, unless that was also unspecified, in which case the runtime will create as many PEs as the system provides.
· Added System::getMaxThreads() that returns the total number of threads (SMP and GPU). It is used in ayudame and extrae instrumentation.
· System::getCpuId is now const.
· omp_get_num_procs returns the number of available cpus instead of PEs. For instance, if the application is run with NX_PES=2 in a 12-core machine where all of them are available, omp_get_num_procs will return 12. gomp behaves this way.