Unfinished work to move normalization into multithreaded c++ code, so that idxstat lengths don't need to be stored while restoring the bamliquidator -c appending functionality.  Didn't work out since I hadn't thought through how to deal with the normalized counts being stored for the cell types.

Just committing for a reference in case I want to revisit this path again.  Next commit will back out a lot of these changes.
Reverted pre-mature plans to move normalization into multithreaded c++
Cleaned up what is left from the last 2 commits so that all tables are stored in a single hdf5 file (which was the good part of the changes from two commits ago)
bamliquidator_batch.py now has a --flatten option to flatten all hdf5 tables