update documentation

README.md

docs/conf.py

docs/cpp/genericio.rst

docs/cpp/library.rst

docs/environment.rst

docs/executables/benchmarks.rst

docs/executables/tools.rst

docs/index.rst

docs/python/class_interface.rst

docs/python/legacy_python.rst

docs/python/mpi.rst

docs/python/readwrite.rst

python/README.md

python/genericio.cpp

setup.py