DataSet: Implement setExtent()
[test] DataSet: tests for setExtent()
DataSet: Use setExtent in write(Values)
PropertyHDF5: Values::write: move size check into ds creation
[test] Property: Add tests to check for size changes in values

This should finally fix issue #36
Use setExtent in all remaining places with true resizing
Replace extend with setExtent in all the code
hdf5::DataSet: remove obsolete extend() function

Also adjust the test
Merge pull request #40 from gicmo/set_extent

Implement DataSet::setExtent to allow true resizing (not just extending) and use it where appropriate