changed install_dependencies_linux to install_dependencies_ubuntu.
updated simple_subset.py for api changes
changed default prefix to ocgis_output.
doc changes
doc changes. modified env.py to note have globals at beginning of file. the "prefix" argument of OcgOperations now reads from env.PREFIX. the selection of the prefix in interpreter.py no longer requires logic.
renamed util/env.py to util/environment.py to avoid confusion. fixed default env.PREFIX value.