Only propagate invalidate if result is valid.
Improved unit mapping creation and added some documentation.
Added documentation to runnable.hpp.
Added documentation to computing_unit.hpp.
Added documentation to computing_unit_manager.hpp.
Added documentation to computing_unit_manager_mpi.hpp.
Added documentation to key.hpp.
Added documentation to task.hpp.
Added documentation to task_entry.hpp.
Added documentation to task_manager.hpp.
Added documentation to task_manager_impl.hpp.
Added documentation to task_manager_mpi.hpp.
Added documentation to computing_unit_manager.cpp.
Added documentation to computing_unit_manager_mpi.cpp.
Added documentation to key.cpp.
Added documentation to task_manager.cpp.
Added documentation to task_manager_mpi.cpp.