cleanup and doxydocs in nr-pixblock.cpp/h

This patch can wait until after 0.42. It accomplishes:
  In libnr/nr-pixblock.h:
    1. interface cleanup of NRPixBlock: size and mode
are now named enums, emptzy is now bool.
    2. changes to function interfaces because of 1.
    3. changed C macros to C++ inline functions
    4. changed clean flag in some function interfaces
from int to bool.
    5. complete doxygen documentation

  in libnr/nr-pixblock.cpp:
    6. adaptations because of 1,2,4
    7. adds assert to ...setup_extern() which remains a
bit ... risky
    8. doxygen docs everywhere except the pixelstore

Thanks for applying.