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.
silenced warning (patch by Michael Krause)
multiple definition issue of projectEllipse2D_FirstQuad and projectEllipse2D solved
added example_nnlsq and improved documentation
region accumulators: fixed histogram initialization when region is uniform (solves #171)
TinyVector: added scalar addition and subtraction
accumulators: improved genericity of kurtosis computation (fixes #172)
Fixed out-of-range exception of 1-dimensional VigraArrays (fixes #174)
finished tutorial page on Python bindings
Merge branch 'rf_hdf5' of https://github.com/burcin/vigra
fixed HDF5File and RandomForest constructor overloading (bug entered in 982ef6eabe9197938ff313d60fd47be8efe917a6)
use "<=" comparison instead of "<" in binary multi-morphology
restored lexicographic comparison in "TinyVector < TinyVector" and added allLess() etc.
added test for chunked array view index access, start completing chunked array view API
fixed typo in documentation
added TinyVector convenience functions and test
refactored and tested chunked arrays
implemented chunk cache by means of std::queue
introduced 'chunk_locked' flag