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.
first experiments with blocked arrays
first version using sparse files (both on Windows and Linux)
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!)
renamed block => chunk (Stuart's suggestion)
gcc compatibility fixes
Merge pull request #1 from ukoethe/master

from master to my fork
first version of merge graph with unit test
changed implementation of hasEdge from map based to ufd based (on the way to remove any map)
changed implementation of hasEdge from map based to ufd based (on the way to remove any map)
made partiotions rep. iterable
removed dynamicEdgeMap from ufd
 implementing strong unit test for merge graph w.i.p.
imporved unit test with very strong 3x3 grid test
fixed bug in merge_graph unit test