Small features: more comfortable thread handling

For a more convenient program handling I suggest the following additions:

1. case: if OpenMP is not activated, and the user enters a thread number > 1, this makes no sense and the user should be informed.

2. case: if the user spares the option '-t' and OpenMP is activated, the thread number should be optimized by OpenMP.

3. the number of threads and processor cores might be displayed additionally for better information.