Fix loading random forests from an hdf5 file into an existing object.

If rf_import_hdf5() was called with an already populated random forest
object, trees and classes were appended to the existing data.
Add support for loading random forests from hdf5 files given by their id.
fixed constructor call in object features test
added unit test macro shouldNot()
added coordinate-wise iteration and test to CoupledScanOrderIterator
CoupledScanOrderIterator: made scan-order and dimension-wise iteration compatible
major clean-up of CoupledScanOrderIterator
gcc compatibility fixes
fixed return type in CoupledDimensionProxy
first experiments with blocked arrays
first version using sparse files (both on Windows and Linux)
silenced warning (patch by Michael Krause)
introduced BlockedArrayTmpFile and enabled use of non-sparse files on Mac
first thread safe version (must be refactored and ported to Linux)
added atomic.hxx
renamed atomic.hxx => threading.hxx and added std::mutex and std::thread to vigra::threading namespace
first truly thread-safe version (need more testing!)
multiple definition issue of projectEllipse2D_FirstQuad and projectEllipse2D solved
renamed block => chunk (Stuart's suggestion)
gcc compatibility fixes