Automatically infer NWChem scratch directory from temporary directory setting

[smcolby]

Currenty scratch_dir is passed as a keyword argument, defaulting to '/scratch', which does not exist on most systems. Instead, we should follow the same paradigm as other temporary file/folder locations in isicle, which is to select system default temporary directory.

Users may set this location by exporting relevant environment variables (e.g. export TMPDIR=/some/path.