returns an existing or creates a new directory wher data for the package
pkgName will be kept. First checks if an environment variable
pkgName.upper() is defined; if not, uses the value of the environment
variables USERHOME (on Windows) or HOME (on Unix) as a prefix.
runs cmdString in a command shell (using os.popen3). Raises an
OSError if the execution generated output on sys.stderr; otherwise,
the output generated on sys.stdout is returned.