issue #450 Made __setitem__ work for multiple image sets in both Measurements and HDF5Dict. Made it possible to initialize measurements with a null hdf5_dict lock if known to be single-threading. Separated code paths for handling indexes in hdf5_dict (because I needed to index multiple image numbers). Null-handling became more consistent as a result and that broke some code that counted on it being inconsistent, so I fixed that. Finally, made use of above to fix mergeoutputfiles and make it run faster.