refactor dependencies

Putting SQLAlchemy in `requirements-dev.txt` is misleading.  SQLAlchemy and psycopg2 are required when using pycsw in standalone mode.

`requirements.txt` are deps needed whether pycsw is standalone or embedded.  This is used by `setup.py` always and is the default / core dependency list.

`requirements-dev.txt` should be just for developer tasks.

`requirements-standalong.txt` should be for standalone (non GeoNode / ODC) installs