Class for parallel reading of partitioned mesh data

Along with commit 4cda158,  member function ``stop()`` of class CPUTime and RunTime were removed, and the calling of the function has been removed in the related files.  
why did you have to change the original way of calculating time, i.e. `(clock()-_start)/static_cast<double>(CLOCKS_PER_SEC)`? The original looks easier to understand.