Possibility to set automatically the number of threads to the max value of number of threads

[tfeuvrier-cs]

Improve command line parameter --parallel-workers : if no arg set, the max number of thread is set.
ex:

• sirius ... -> 1 thread (default value)
• sirius ... --parallel-workers=2 -> 2 threads
• sirius ... --parallel-workers -> max number of threads

[tfeuvrier-cs]

@savmickael : code modified and tested.

[tfeuvrier-cs]

PR Merge in develop.