handle missing input files
Merge pull request #484 from ged-lab/fix/assemstats3-nofile

handle missing input files for sandbox/assemstats3.py
update doc/install.txt to use python2 explicitly
further modifications for python2 install
updated commands vice @mr-c
Merge pull request #485 from ged-lab/update/use-python2

Be explicit about python2/pip2 in the install docs.
slim down the PyPI distribution